Straight weight/multi-vis oil

[JasonB]

Possible dumb question, but why is straight weight oil recommended for marine engines (at least older ones). I have a fresh rebuilt '76 Mercruiser 302 V8 (about 20 hours so far) that I have run the recommended st. 30 weight in since she was put in. Would a good 10w30 work as well? I have high oil pressure and am wondering if dropping to a 10w30 might lower is a lb or two (high pressure due to high volume pump).

 

[Boatist]

Re: Straight weight/multi-vis oil

JasonB<br />I ask same question of OMC factory rep back about 1985. His answer was that mutli vis oils break down faster at full throttle than straight weight. Also both boating and auto industry says 10-40 can cause rings to stick. He said ok to use synetic if can find it in a straight 30.

 

[dhammann]

Re: Straight weight/multi-vis oil

Synthetic oil is Promised technology, SAE 30 wt is Proven technology. If the clearances on your rebuild is at factory specs you should be OK with 30wt. 10-30 oil is actually 10wt oil! Multi grade oil is an automotive compromise for varying climatic conditions and not usually applicable in marine use. Always allow your boat engine<br />to warm up before going WOT. This will reduce the stress on your oil pump.

 

[petryshyn]

Re: Straight weight/multi-vis oil

JasonB<br /><br />How high is your oil pressure? Have you checked it with a mechanical master gauge?<br /><br />As Big Dee says, low end multigrades lose there viscosity rapidly when used in a harsh enviroment. Marines engine certainly fall in that category. A cheap 10w30 will soon become a 10 to 20 weight after much use. Higher end synthetics last longer as a true multi-grade, but are they worth the price. I wouldn't hesitate to use a straight weight if the manufacturer recommended it. Many small engines (which run in the absolute worst conditions) call for straight weight, especially if there is minimal climatic changes