Re: Engine Oil ???
Latest Mercruiser fuel and oil recommendations:
http://www.mercurymarine.com/serviceandwarranty/mercruiserfaqs/fueloil.php
Oil chart from above link:
The reason they SPECIFICALLY DO NOT recommend the multi-viscosity automotive oils (see the NOTE just below #5) is because they are formulated from the thinnest base with plasticisers added to give the higher weight when hot. For example a 10W-40 is a 10 weight base oil.
As you shear enough of the plastic molecules under load, the oil slowly reverts to it's base weight. A heavily loaded marine engine becomes very unhappy when you start running thin oil in it .......
Nev'r Enuff, just as a sidenote, I also have a 1986 Searay 300 with twin 350's. I recently had to replace a single piston in the starboard engine because of detonation damage. Engine has about 650 hours on it, had 150 PSI in all other cylinders. Internally it looked brand new. Bearings and cylinders looked new, no wear rings at all at the top of the cylinders. Replaced the bad piston with a new standard piston and it runs like new. i have all the receipts from the marina where it was maintained by the 2 previous owners, it's had the factory Mercury oil in it since new, changed every year.