What is the absolute best oil to use in Mercruiser 600's? We use Mobil 1 in the other 3 boats (Merc 5.0, Merc 502, merc 496's)

Everyone will agree that mobile one is good. But if it is labeled by the API certified for a passanger car, I would not use it. It does not have enough anti-wear and corrosion resistant additives. If price was no object, I would go with the "Joe Gibbs" labbled oil. Since I cant decide myself, I wound up running Merc's Semmi Synthetic.
Jr.

Your manual states Kendall 20W50. That would be a good choice for a conventional oil. Otherwise, I would run 20W50 Mobil 1 V-Twin.

we want to use a full synthetic

Then Mobil 1 V-Twin 20W50, Red Line 20W50, Amsoil Racing 20W50 would all be good choices. Mentalpause who has the first F-4 Sunsation is running V-Twin in his 600's with good results. Carries a ton of oil pressure after running hard compared to other oils.

Im at Mobil 1's website and I dont see the V-Twin or any 20W50 on there? Thanks for the responses!!

Edit: Nevermind, found it in the motorcycle section

Mobil 1 15/50 is what I use in my 525SC and change it every 25 hours. The old version(less $$) is back at Walmart in the 5 quart jugs.

Correct, it is a motorcycle oil and not subject to the latest emission requirements. We have run it in several race cars with excellent results.

Thanks again, Didnt see it at walmart. Would most auto stores carry it?

Mobil 1 VTwin 20W-50 or the new Mobil 1 Silver Cap 15W-50. Read Hydrocruiser's thread.