www.iFourWinns.com

Years ago [1996 I think] Consumer Reports (for what its worth) did a pretty exhaustive test comparing syn vs conventional oils and 3K change intervals vs 6K change intervals. The test vehicles were full size Chevy cabs with 4.3 V-6s. They were ran around the clock and then the engines were taken apart and wear measured...and...there really wasn't much difference if at all...but that's cab use...minimal cold starts...stop and go...different than boats that run cold coolant temps but high rpm use and probably have higher valve train temps by a lot...so like you said it's hard to get the right information. I used syn 75W-140 gear oil in my 98 Jeep's diffs and even though they are known for noisy diffs they have been quiet through 126000 miles of use...I will continue this with my 07 Jeep and have also done it with our 98 Subaru...I do believe in syn lubricants for certain applications but Volvo has not really explained why the recommend a syn straight weight...

Having said all that....I will probably run any newer vehicles we own on syn motor oil....I have to use 5w-20 in my 07 Jeep Hemi...so I use Mobil 1 for that...

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

My VP manual states that with regular oil you can go 100 hours between changes and 200 hours with synthetic. This doesn't mean much to me however as I only average maybe 50 hours a season.

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2008 FW F204 - 5.0 VP "Stand On It"
Former Boat: Larson 186 LXI - 4.3 VP

Now, just to make things even more interesting everyone, Mercury recommend MerCruiser Full Synthetic Engine Oil - SAE 20W-40. Now this makes sense to me for a multi vis synthetic oil, and is definately more readily available than the single vis oil Volvo recommend for exactly the same engine.

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Alec

2006 Horizon 190
VP 4.3GL/SX, Sunsport