Preheating is a deep subject, way more concern than oil alone. No, improper preheating will not pull you out of the sky but doing it right may stay off the overhaul job...
Here is what I wrote on the Cessna 120/140 forum, one of the more knowledgable guys and mechanics said it was pretty spot on...
Preheating is far more complex and in depth topic than i undedstood it to be! Its not really about having goey oil.. its bearing clearance in the bottom and keeping pistons from scoring up top and condensation issues...
From what i can gather under 40 its time, but wouldn't hurt at any temp though in actuality, and its a mortal sin in the 20s and under to not preheat...
So heating bottom end is vital as cold oil isn't as much an issue w multi viscosity aviation oil as the fact that the aluminum block and steel crank don't proportionately shrink with the temp, so the bearing clearances are squeezed below their factory designed minimums with the aluminum shrinking more than the steel to the point its so tight that their is no room for oil and metal is kissing metal... so gotta make sure bottom is warm. This can't be done with a 20 minute blast...
Now up top our piston walls are not parrallel with each other at cold temps, instead there is a slight conical shape to our bore with the top being narrower. This is built in by design because air cooled cylders dont hold the same temp at bottom as they do the top under normal operting condition. So our cylinder walls lose their cone shape as they reach operating temp and their tops are hotter than their bottoms... so started cold, that piston is pushing up into a cylder too small at top for low wear operation. One engine "expert" i read said a single cold start can put as much wear on cylder walls as 500 hours of cruise flight...
Now condensation... the ford vs chevy argument in preheat is: on all the time vs on just as long before you need it as neccesary..the issue is an engine that's toasty down bottom but freezing up top is somewhere in that engine is an area at dew point now... engines are humid inside, a byproduct of combustion and some blow by and such... so theres risk with a continually heated engine if theres a significant temp differential within it of creating condensation to sit on engine parts-not good. Thus some preheat just long enough to get to temp, length dependent on environment and equipment capabilities, minimizing the time that condesation is in there on parts that are daily loosing thier oil coating....
I think the truth is in the middle, if you can heat your engine to an even amount and above dewpoint danger, preheat around the clock. If you can't, preheat when necessary just before flight but long enough to get her warm in and out. And if one does the "just long enough" method and cancels flight, better to try to get even temp on engine (covering it all up) and let preheat stay on till next flight however long that may be, as cooling it again brings it through dew point after being unused so not getting coated in oil again, so best to leave it on till next flight. I understand it may not always be possible- these are just theoretical ideals.
I'm going to sew her a cowl cover custom to wrap cowl including the front, and make propeller covers as well, as the prop and spinner act as a huge aluminum heat sink for the engine. A heat sink is the metal cooling device used on electronics to passively cool them by radiating off heat, prop does same thing to our engine in preaheat... I don't think the prop covers are a necessity but I view the cowl cover or wrapping it with blanket is...
So long and short of it: its never really too warm to preheat, when in doubt do it. A preheat thats truly effective will get top and bottom warm whatever method one chooses. Be conscous of condensation and find a way to best mitigate its danger. There's also a thread on engine dehydrators on here, thats not an entirely unreleated topic...
Again this is off google research (though a lot of it) and triangulating forum arguments on method not first hand knowledge yet. Take it for what its worth...