So, for all you motor heads, me included what types of oil do you use on your big blocks. I have a set of 496's and have read 3 different things from Mercruiser. 20w40. or 25w40. First they said full Synthetic and then blended. I have always used Royal Purple 15w40 but would like some other choices. Still change every 50 hours. In the generator straight 30W Synthetic.

Any 20w-50 will work....I like Kendall

Can't go wrong with a good quality 20W-50 form Kendall, Castrol, Valvoline or Pennzoil. If you want to spend more money look to synthetics. There is a good thread called "marine lubrication" that has a lot of good information.

I was looking into the Royal Purple full Synthetic 15-40. This is close to the 25-40 Merc recommends. I run my boat 90% at 3000 RPMs so a very heavy weight oil won't be needed. I looks like Sierra is the same as Mercruiser oil but different label?

We've always used M1 15w50. Good oil at a good price. Just bought six 5 qt jugs on sale at Walmart for $21 a jug.

I was looking into the Royal Purple full Synthetic 15-40. This is close to the 25-40 Merc recommends. I run my boat 90% at 3000 RPMs so a very heavy weight oil won't be needed. I looks like Sierra is the same as Mercruiser oil but different label?