Well,here goes,,,,,,,,,,,,,,,,,,,,the black oily blood bath.
The SAE number relates to the viscosity at a standard temperature and,as you say,is selected with reference to the bearing loads in an engine which may,or may not,be a consequence of close tolerances. Just think cam shafts.
A quick & dirty fix for oil loss past the compression & oil scraper piston rings may be a higher viscosity oil vis a vis the SAE60 spec. on the 3200 engine. (BTW,isn't the 4200 just a stretched version of the 3200)
The second number,the winter grade,such as 5W is a measure of the ability of the oil to flow (more correctly,it's ability to be pumped) at low temperatures. Do not confuse it with the SAE number.
Synthetic oils are just the chemicals which replace mineral oil,originally used in aviation for their superior resistance to heat stress. The quality of the oil should not be confused with the properties of the additives used in the lubricating oil formulation. Ashless additives are now in universal use so we don't see those chalky deposits in the combustion chamber. Both mineral & synthetic oils contain no ash,it was the older additives which contributed the ash. Oxidation inhibitors have been important additives for many years with the advent of higher engine temperatures. Synthetic oil based on esters are inherently more resistant to oxidation,however,it's the additives which do the work.
Viscosity index improvers based on polyolefins created the multigrade oils such as 40-10W etc. way back in the 60's. These polymers have been refined to the point where they are extremely stable and it is now possible to keep the viscosity index in specification for the life of the oil. The polymers were susceptible to shear stress but the new additives are virtually indestructable.
Engine manufacturers are not god-like,they don't know anything more than the average Joe in the car industry so they take the advice of the oil & additive experts. These guys get their knowledge by wrecking a lot of test engines before they can formulate an oil meeting the strict SAE requirements. Anything you buy in a pretty retail pack is backed by a lot of expensive test work,there aren't any shonky blends. So I'm saying that you can use any brand and you don't have to stick to the manufacturer's recommendations as long as you make a sensible choice as a replacement grade. I have used SAE 30 in the 3200 engine with no adverse consequences.
And the reason is that modern oils are formulated to flow more quickly through bearing gaps rather than be biased towards a "cushioning" effect by establishing an indestructable boundary layer,they seem to work better this way.
End of sermon,you may all now take a bath.