First, there is nothing in the standards using the term "weight" for oil types. There is only an SAE standard in which the oil's viscosity is defined by an SAE value, typically in the range 5 to 50 for aircraft engine oils (lower being less viscous thus better in cold ambient starting conditions, and higher being more viscous thus better in hot ambient starting conditions).
Aeroshell uses its own numbering system for its single-grade oils, in which the number is roughly twice the SAE standard value (but not always). So, Aeroshell 100 is SAE 50 viscosity oil. Some people will call that "hundred weight", and some people will call that "fifty weight". Whether that means to
you that it's "hundred weight" or "fifty weight" is something you have to work out to yourself. In addition, Aeroshell's number system is not hard and fast -- their SAE 30 oil is "Aeroshell 65".
In addition, the "W" in those oil titles has nothing to do with viscosity, but rather indicates that the oil includes an ashless dispersant additive. This is the big difference between "regular" oil we normally use, and pure mineral oil with no additives at all generally used during cylinder break-in. So, Aeroshell 100 is their pure mineral SAE 50 viscosity oil with no additives, and Aeroshell 100W is their SAE 50 viscosity with the ashless dispersant additive. Aeroshell further differentiates them by putting straight mineral oils in white containers, and ashless dispersant oils in black containers.
Likewise, Aeroshell's multiviscosity oil is their 15W-50. The 15-50 indicates first that it function properly in conditions as cold as those requiring an SAE 15 viscosity oil, and from there anything up to conditions hot enough to require SAE 50 viscosity oil. As always, the W tells you it has the ashless dispersant additive.
Finally, Aeroshell also premixes the Lycoming LW-16702 required in certain Lycoming engines (mainly the
0-320-H and O/LO-360-E types -- see Lycoming SI 1014M for more on that). Aeroshell indicates the presence of this additive in their single grade oils by a + in the name, e.g., Aeroshell 100W+. This additive won't hurt engines that don't need it (although they interfere with break-in, which is why Aeroshell doesn't make a straight mineral oil with it), but it's essential for those that do require it. It's also included in their multiviscosity 15W-50 even though they don't put the + in the name.
And just for a bit more fun, Aeroshell 15W-50 multiviscosity oil is has a semi-synthetic stock base, as opposed to their single grade oils which are all made from a 100% "natural" mineral oil base. For comparison, Phillips' X/C 20W-50 (covers the viscosity range from SAE 20 to SAE 50 and has an ashless dispersant) comes from a 100% natural mineral oil base, but Exxon Elite's 20W-50 is semi-synthetic, like Aeroshell's multiviscosity oil. And there is nothing in the numbering system to tell you that -- you have to either be familiar with the products or read the label to know (or know that the price per quart is nearly double for semi-synthetics than for all-natural mineral based oils).
Finally, there's the table in that SI linked above regarding what SAE viscosity value you want in your Lycoming engine depending on ambient starting conditions. No doubt TCM has a similar instruction with similar table for those with Continental engines.
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