I've read all the posts on this subject, and still have a question. In a car engine, one does not mix weights of oil. (Nor in a trans, or differential, for that matter.) How about the Stanley crankcase? Does it matter? Secondly, if I were to use synthetic, what weight? 90, which is commonly available?

Ianc.
The wet Stanley engine used Steam cylinder oil to lubricate the engine. Mainly for two reasons, condensed steam (water) can get into the engine when the piston and valve packing glands leak. There is no packing on the valve guide casting. The oil gets foamy, but with steam cylinder oil it protects the engine very well. The other reason is the seals on the baffle plate and rear end plates. Same as a model T. they use steam cylinder oil in the rear end as it won’t leak by the felt seals on the wheel bearings.
That said any good gear oil would lubricate the engine. Most synthetic separate the water very fast and will settle to the bottom of the case. At one time I did install for some one using synthetic gear oil a drain plug in the copper wrap to drain water that would acculturate. I don’t know how it affected the rear end seals as it was a condensing car.
If you use oil other then steam cylinder oil you want to make sure it has an additive to protect against corroding aluminum copper and bronze. The baffle plate is aluminum and the valve guide casting is bronze and of course the jacket is copper.

Rolly

Sulfur is a common oil additive[at least was],,,and when in contact with water,,,,,combines to form sulfuric acid,,which attacks most everything ,,including ones pocketbook !!!,,,,Cheers,,Ben