Dominator 15-50 is the best stuff out there. I would try and use a larger filter if you can fit it and change it frequently for the first couple of oil changes. I recommend staying away from racing filters they pass too much junk. Let us know what filter you have and how much room and we can possibly cross it to a larger version.

just messing with ya

I use Rotella in the motor, but, I have used Amsoil in the outdrive and in several tow vehicles, works very well, and I have had several other friends and co workers have great results with it, however as an earlier post stated they DO make several different marine oils. The Dominator stuff is supposed to be VERY good I am told.

It appears you are all more expert than me on this matter. I can only speak from experience.
Interestingly I was told by a reputable source that because of the less friction with the use of synthetic oil, roller lifters actually slide instead of the rollers turning as designed. I guess that are a lot of people on both sides of this argument.

Also I had a 1996 Mercruiser 350 Cubic inches that I beefed up with an Eldebrock Performer intake manifold and cam that would not hold decent oil pressure using synthetic oil.

Also I had a ZZ502 502HP (bad choice for marine application) that I had the same issue with.

Also back in the day I had a 66 Mustang GT 289 four barrel which I replaced with a 289 Cobra dual aluminum high rise, solid lifters, and centrifical advance distributor which hated the first synthetics that came out.

The best oil for me always has been non-synthetic Mercury oil in terms of holding oil pressure after the motor has been beat on. My Mercury guy tells me that Mercury beefs up this oil with some descent additives.

I have two similar motors as yours : 540s, dart heads and brodix intakes with EFI and a pair of vortech chargers pushing about 6 lbs boost.

They have run Amsoil 20w50 from day 1 (after break-in) and have had no issues whatsover. Hours are 175 right now.

when you say more suit than usuial,do you mean black on transom,because that is probibally from being rich ,not from different oil.i know a lot of people use synthetic oil in their boats,i use brad penn 20-50 in my boats and i highly recomend it.

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I use Joe Gibbs oil or Kendall 20w50 w zink

+1. I did run amsoil dominator and now run Joe Gibbs Hot Rod Oil per my builder. He said he runs it in all his high performance motors whether it be drag, circle, pulling, or marine. Supposed to be high in zinc

Why is zinc important? Does it also apply to everyday engines?