This is from one of the Vintage enewsletters I get New Tech Tip: Another Reason to Change Your Oil! Our motorcycles don't have external auto-type alternators. Instead, vintage Hondas have a fixed, internal stator that lives in a "wet" environment...constantly bathed in engine oil. This is a good design in that the oil bath keeps the stator windings cool. But, worn out motor oil is thought by some to be a contributing factor in premature stator failure.
Here is a link to the tech tip Like I mentioned, it's a Vintage enewsletters and I honestly don't know how many bikes today have wet stators if any. I've replaced two GL stators both from bikes that had been sitting for a long time. Both has less than half their output and both had serious deterioration to their isolator coatings. It could have been from age, being stagnate, being in old oil or any combination.
It is thought that the acids can deteriorate the thin, insulating coatings on the windings causing them to fail but acids don't attack your plastic battery case and the coating on the insulator is also made of plastic... Heat is the real culprit that kills alternators... you can tell that the windings get hot by the color of the insulating plastic which turns dark because of the engine oil... After 50 K miles the engine oil has turned Mr.RC45 stator brown but it still puts out the volts...