Oil

straight or multi weight oil which do you use

  • Straight weight

    Votes: 15 19.7%

  • Multi weight

    Votes: 61 80.3%
  • Total voters
    76
Whitney

Whitney

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Whitney
This is not your average oil thread, and please don't turn it into WWIII.

Do you run multi weight, or straight weight oil in your piston powered plane?
No need to say a brand name, what weight you use, just yes or no, multi or straight weight.
Got into an interesting conversation with 2 pilots this afternoon on this topic.
 
Whitney said:
This is not your average oil thread, and please don't turn it into WWIII.

Do you run multi weight, or straight weight oil in your piston powered plane?
No need to say a brand name, what weight you use, just yes or no, multi or straight weight.
Got into an interesting conversation with 2 pilots this afternoon on this topic.
Click to expand...
I ran straight. It’s what they recommended, so that’s what I did. Never bothered to pro and con it.
 
I feel like there's a drawn line, somewhere appx at the US California/Oregon border, above which you run 20/50, and below which you run W100.
 
I ran straight weight for many years in my planes, changed to multi weight when I bought my current plane 14 years ago, as the previous owner had been using it, and I figured why not give it a try. IMO both work fine.
 
I run Phillips 20/50 + CamGuard. Here in Florida I'm sure I could run straight weight, but I don't see any advantage to it. My plane's POH warns about monitoring oil temp for excessive cooling during long descents, and it seems to me using multi-weight mitigates that concern a bit. Besides, if I decide to visit relatives in a colder climate, I'd rather not have to change my oil just to make the trip.
 
I use 25W-60 in the P&W radials.

When I owned my Baron, I started with a multi, but switched to a straight weight. I found that the oil analysis looked better with the straight weight than the multi, largely because while I flew it around 100 hours per year, it would sit for extended periods without use between trips.
 
Fearless Tower said:
I use 25W-60 in the P&W radials.

When I owned my Baron, I started with a multi, but switched to a straight weight. I found that the oil analysis looked better with the straight weight than the multi, largely because while I flew it around 100 hours per year, it would sit for extended periods without use between trips.
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That brings up a point. Hours vs the Calendar.
 
For anyone who didn’t already know, the point of multi-weight is it works well in cold or hot conditions. The lower number is how it flows cold the higher number is how if flows hot basically.

with straight weight you might need to run a lower weight oil in the winter vs summer, multi eliminates that problem. I don’t really understand what the downside to that could be.
 
The only time I used straight weight was breaking in a fresh Penn Yan overhaul (to new specs) of my O-320... per Penn Yan breakin instructions.
 
I don't own an aircraft. I do run straight 30w in the lawn mower...because I don't ever use the lawn mower in the winter. In the snow blower, I run 5-20W or something synthetic, because I do run that in the winter. There are reported advantages of straight weight oil over multi, I don't know if they're true. The only advantage I've heard of for the variables and especially synthetics is cold weather performance on startup. Variable synthetics absolutely do start easier in the winter. My theory is that straight weights may provide better corrosion protection for engines only run in the summer...but that's just a theory.
 
Country Flier said:
People still use straight weight??? These same people seek out black-and-white TVs instead of color? Use monochrome monitors? Land lines with rotary dials?
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There are 4 other planes, besides mine, that land at my farm as home. 1 of them still uses straight weight, and is what sparked todays convo. Last week I changed the oil in mine, today 2 pilots who are friends, and based here, were out changing their oil. They are polar opposites, and best friends. One runs multi grade, collected a sample for analysis, and cuts open the filter for a look.
The other uses straight weight, never samples, and never checks his filter. Both have flown a long time, and get good long engine life.
 
cowman said:
For anyone who didn’t already know, the point of multi-weight is it works well in cold or hot conditions. The lower number is how it flows cold the higher number is how if flows hot basically.

with straight weight you might need to run a lower weight oil in the winter vs summer, multi eliminates that problem. I don’t really understand what the downside to that could be.
Click to expand...
Generally a good multi-weight oil is the best option. That said, it is not true that a straight-weight oil and a multi-weight oil behave the same at the same temperature. The multi-weight oil has viscosity index improvers added to allow the oil to flow differently at different temperatures. These additives are easily sheared by the piston rings, reducing the viscosity of the oil on the cylinder wall, thinning the multi-weight oil along the cylinder wall (we are talking about a very small amount of oil on the walls being sheared and thinned with each stroke). A straight weight oil does not have the VI improvers and does not get sheared to a lower viscosity by the rings. What does this mean?

With multi-weight: In most cases it will mean better cooling and less cylinder wall drag, as the sheared VI's locally around the rings thin the oil out, produce less piston resistance, and cool the rings better (particularly the top ring).
With straight-weight: Less cylinder wear, probably hotter rings, and more drag from thicker oil along the rings. Less oil consumption. It does not shear down to a lower viscosity within the local area of the rings.

So, use multi-weight, unless you find that you're consuming more oil than you'd like. In that case, use straight-weight, and you will likely see less oil making it by the rings, and thus lower oil consumption.

For small engines (lawn-mowers, etc), I generally run SAE 30, and have definitely noticed reduced oil consumption compared to 10W-30. With the rental airplane 172 fleet, I ran multi-weight.

SAE Technical Paper 910742 goes into this with far more detail.
 
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Living in the north central US creates some large temperature fluctuations. Because of this, most of the airplanes are running cultivating viscosity oil to make things easier. We do run straight weight oil in the helicopter and in the float planes, which don’t fly in the cold months.
 
Whitney said:
never checks his filter
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The rest I can forgive, but that is dangerous. I wouldn't fly in that plane. How does he know when the engine is trying to tell him it's coming apart? Does he just assume it's definitely going to make tbo?

Engine oil is massively over-thought. Any oil is better than no oil. On the farm I have gotten to the point that everything from the lawn mower to the pickups to the semis to the combine get the same 15w-40 out of the bulk tank. It's designed for the big diesels and its close enough for the gas engines.

I like multi-vis in the plane so I don't have to worry about stocking multiple grades for winter vs summer. One of my missions is illinois- Florida in February, what straight weight would I run for that? We've had 20 degrees days and 80 degrees days in the same March.

The shop in GA put straight w100 in. It was interesting as it acts different than my normal 20w-50. I will say I didn't notice it getting thinner over time like the multigrade, but I also only ran it 25 hours vs 50. The multigrade after 50 hours is still well within spec, and I would run it longer if it wasn't so loaded with lead. I think unleaded fuel will make 100 hour oil change intervals realistic..... and guys will still do it at 25.....
 
jesse said:
Generally a good multi-weight oil is the best option. That said, it is not true that a straight-weight oil and a multi-weight oil behave the same at the same temperature. The multi-weight oil has viscosity index improvers added to allow the oil to flow differently at different temperatures. These additives are easily sheared by the piston rings, reducing the viscosity of the oil on the cylinder wall, thinning the multi-weight oil along the cylinder wall (we are talking about a very small amount of oil on the walls being sheared and thinned with each stroke). A straight weight oil does not have the VI improvers and does not get sheared to a lower viscosity by the rings. What does this mean?

With multi-weight: In most cases it will mean better cooling and less cylinder wall drag, as the sheared VI's locally around the rings thin the oil out, produce less piston resistance, and cool the rings better (particularly the top ring).
With straight-weight: Less cylinder wear, probably hotter rings, and more drag from thicker oil along the rings. Less oil consumption. It does not shear down to a lower viscosity within the local area of the rings.

So, use multi-weight, unless you find that you're consuming more oil than you'd like. In that case, use straight-weight, and you will likely see less oil making it by the rings, and thus lower oil consumption.

For small engines (lawn-mowers, etc), I generally run SAE 30, and have definitely noticed reduced oil consumption compared to 10W-30. With the rental airplane 172 fleet, I ran multi-weight.

SAE Technical Paper 910742 goes into this with far more detail.
Click to expand...
You said “…With straight-weight: Less cylinder wear…” How much? There’s going to be a lot of variables obviously. But let’s say, everything else being equal, that it would typically make the difference between needing a top end overhaul or not before reaching TBO?
 
10w-40
aeroshell-peo-sport-plus-4.jpeg
 
cowman said:
For anyone who didn’t already know, the point of multi-weight is it works well in cold or hot conditions. The lower number is how it flows cold the higher number is how if flows hot basically.

with straight weight you might need to run a lower weight oil in the winter vs summer, multi eliminates that problem. I don’t really understand what the downside to that could be.
Click to expand...
luvflyin said:
You said “…With straight-weight: Less cylinder wear…” How much? There’s going to be a lot of variables obviously. But let’s say, everything else being equal, that it would typically make the difference between needing a top end overhaul or not before reaching TBO?
Click to expand...
I doubt it'd be noticeable as to when you'd overhaul. This also assumes you never start the engine "too cold" for the straight weight oil, if you do, the multi-weight's advantage at cold startup will easily exceed the cylinder/ring wear differences between straight and multi. Truth be told it's hard to make precise statements about the differences because it'll vary depending on operating conditions and engine specifics.

BobIsTheOilGuy is a great forum if you want to get deep into the technical details of different oils. Anyhow, you will find the experts there don't even agree on this stuff exactly....Because quite frankly...there isn't a clear answer given all the other variables at work!
 
jesse said:
I doubt it'd be noticeable as to when you'd overhaul. This also assumes you never start the engine "too cold" for the straight weight oil, if you do, the multi-weight's advantage at cold startup will easily exceed the cylinder/ring wear differences between straight and multi. Truth be told it's hard to make precise statements about the differences because it'll vary depending on operating conditions and engine specifics.

BobIsTheOilGuy is a great forum if you want to get deep into the technical details of different oils. Anyhow, you will find the experts there don't even agree on this stuff exactly....Because quite frankly...there isn't a clear answer given all the other variables at work!
Click to expand...


When its nasty cold, drain the oil, preheat on the stove, and pour it back in, then start the engine.
 
Whitney said:
When its nasty cold, drain the oil, preheat on the stove, and pour it back in, then start the engine.
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That is a lot of work, lol...Also nothing like waiting for oil thicker than molasses to drain out of an airplane while you're freezing your ass off. I would use an electric block heater, and if that weren't available, a propane powered solution.
 
Hmm, I'll bet bacon and eggs taste real great at the Whitney residence.
 
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