I bought a 88 Crestliner with a 4.3 in it and went through the same thing. As SAE is no longer a rating they use. Plus new engines have different cam shafts and lifters and use different oils. I ended up with 15W40 Rotella oil in mine. As Mine said 10w for cold weather 20w for reg temps amd 30 for hot weather. Been very pleased with how it's worked.