I have a 1990 Regal 233 XL Ambassador (Cabin Cruiser Type) - carburated Mercruiser 5.7 (350 ci). I live near the border of Missouri & Arkansas, all lake water use. I would like to know what weight and brand of oil to use. I've had a number of people tell me straight 40 or 30, 10-40 or 10-30 and use mobile1. Mostly I heard straight 30 for Northern boats and straight 40 for Southern. Thanks to all that reply!