[hellosailor]

Bill, 10W40 should be stocked at any auto parts store. And any gas station. If you don't see it and don't need to do the oil change today, any service station can order it in for you. That might mean buying a case (12 cans) but wtf, it won't go stale.

Usually you can go below the ##W end of the rating, a lower number just means easier starting in cold weather. And the higher number reflects typical operation, that's more important. If your engine calls for 40 and you use 30, it is under-protected in high temperatures. If you use 50, the oil stays too thick and you lose some economy and power. But if you are worried, the right oil certainly can be ordered in for you. Check a price versus mail order or Amazon just to make sure you're not being taken advantage of on that.

 

[hellosailor]

API Engine Oil Classification
SAE International

Actually it is easy to get more information on oil specs. The thickness ratings are from the SAE, they've got lots of files available. The complete rating, including spark-vs-combustion use (gas/v/diesel) and the alphabetical rating (i.e. "SF") some from them.

Any engine oil that was "top quality" 30 years ago, would be considered junk today. That's what the alphabetic ratings are about.

beaer in nind that any 10W40 oil actually IS a 15W40 oil, as the temperature comes up from the cold test point to the warm test point. The curves will meet and combine. So if your engine calls for 10W40, but you're using it in the tropics and it will never see sub-freezing temperatures? Yeah, 15W40 "is" the same thing for all intents and purposes.

Or, you get a bottle of 5W40 and a bottle of 15W40 and combine them. Might not be perfect, but the additives do average out. Beats using Crisco anyday.