1982 Mercruiser 470 engine. My manual says use "SAE 40" oil, a straight-weight since I'm in a warm climate (over 50F temp. SW Fla). Also says optional 20w40 or 20w50 is possible but not preferable (Aslo hard to find). Strraight SAE 40 is not turning out to be very common, even at stores like West Marine. Any thoughts on best place to find it? Or does it really not matter much and I should just go with 25w-40 which is east to find.