There are a lot of different types of oil produced, and sold, many at lower, older, standards levels, than the current maximum (
API SN).
Here is the link to the API oil rating PDF file, which explains it all, in detail:
API 2010 Oil GuideIt is still possible to buy some of the older standards, and possibly save some cash in doing so. I visited my local
Mills Fleet Farm store, today, and I scoped out their oil aisles. I found they had oils labeled for diesel engines, automobile engines, motorcycles, ATV's, and snowmobiles (I know the diesel oils have higher levels of detergent, but I doubt the others differ aside from their API ratings/compositions). I examined the snowmobile oil, in particular: it was full synthetic (I need to go back and get its name, so I can verify on its MSDS sheet that it is a full synthetic, and not a highly refined mineral oil), 5W-30,
API SJ rating (for engines from 2001, and older; no friction modifiers, so it won't affect a wet clutch, and the normal, automotive level of detergents, unlike diesel oils). It was just under $5/quart, while the
Amsoil synthetic, was >$10/quart...
I am not saying that all synthetic oils are the same. I am not saying that this particular brand of synthetic, snowmobile oil, is equal to
Amsoil. I am saying that it was billed as a full synthetic oil, with an API rating of SJ: this oil is appropriate for all engines from 2001, and older, but not my '03 car. For me, the
Rotella T6 (
link) will still work in all of my 4-stroke engines (SM: 2010, and older engines), which is why I won't be stocking up on the snowmobile oil, because I would have to have two types on hand, for all of my vehicles.
Anyway, if you want inexpensive synthetic oil, that exceeds the API rating for your vintage bikes, you may want to shop around, and see what you can find. You might find a better oil, for less money. By the way,
Mills Fleet Farm has the
Shell Rotella T6 Synthetic, for around $4 less, per gallon, than
Wal-Mart: $22/gallon, as opposed to $26/gallon; they also sell it in a box of three, one gallon bottles, for $65, saving a buck off the individual bottle price. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII