Have a 540 bulldog with blower , 6lbs boost, intercooler, 400 hrs on motor, switched from Merc oil to Amsoil............ 1st time out my friend running next to me notices a slight smoke plume trail behind my boat, after pulling boat out I notice more soot than usual on the transom, What gives? Any one ever have these symptoms right after changing to a synthetic oil?.............. checked oil level after the day of running no noticable amount of oil used

Someone else on here may give you a more exact answer but I do know that Amsoil has many different kinds of marine and non marine oils that get pretty specific on which ones work best for certain engines ie high zinc, low zinc, roller lifters etc. Seeing that you are running a blower I would get ahold of a Amsoil rep that knows his stuff and see what's best for your application.

So why did you switch? I mean, was there a reason?

seen it happen in cars before...400 hours is equivalent of a high mileage car...when you switch to synthetic,they start pumping oil...you switch them back to regular oil and the problem disapears

Which Amsoil product are you using? They make more varieties than just about anyone. I have switched high mileage vehicles from refined oil to synthetics with no problems. The only time you may have a problem is when you have an excessive amount of sludge and varnish built up from a low detergent oil and switch to a high grade synthetic which will loosen up the sludge and varnish and open up clearances. I once worked on an old 265 Mustang that had so much sludge that oil couldn't get to the valves. Once I cleaned it up it smoked like a mosquito fogger. Tell us the product code and I can tell you if you are using the ideal product for your application.

I switched at the recommendation of a good mechanic thats been working on my boat, Amsoil Dominator racing oil 15-50. I will run the boat this week end and see if it gets any better. If it's just cleaning out the crud I would think that would be a good thing. Right?

depends on the ammount of crud. If there is alot it can clog the oil pickup (worst case senario) but if you've been good on oil intervals, you should be fine

Thisislivin is very correct about his statements as the esters used in the syn oils help remove / clean certain deposits & sludge that was built up. Overtime some older seals that are not as plyable as they use to be can leak because of the cleaned up deposits & sludge around them. This cleaning process does not happen right away either.

260 or 289

I stand corrected it was a 260, 265 was Chevy.