Hi I live in Michigan and am getting ready to fire up my old 1989 chaparral 7.4 Mercruiser. The boat has been sitting for 6+ years. I was gong to run a cheaper oil just for the start up then put in some Mobil 1. Any suggestions on what weight to use? I was thinking 20w-50

For a cheap oil i run delo 15 40 it has all the good stuff in it for flat tappet cams. Alot of over the road guys run it.

Rotella T.. Run it in everything..


John jr

No need for a overly viscous oil like a 20w-50. All that's going to gain you is poor start up
Lubrication and loss of horsepower. 10w-40 or a 15w-40 diesel oil works very well in a merc big block.

My buddy has an older Wellcraft with a 7.4 he runs the cheapest SAE 30wt he can find. Motor has over 700 hours on it and the only thing he has done to it literally is change the oil and a set of manifolds last year. The motor does have a hard life cause he's always boating with guys that have fast boats and often ends up doing some long tows across the bay at 30 mph...

99 cent WalMart oil or $12.99 a qt VTwin, you will never know the difference. Basic engine, doesn't matter.

Maybe in a stock Mercruiser 7.4, however this statement is false regarding racing engines.

If you will look more closely...."Basic engine, doesn't matter" and this is correct. No racing motor here.

I did, and as such clarified. I like to make sure clarification is made if someone else happens upon this thread. Oil is one of the least understood topics here on OSO. Fwiw

It is least understood because it does not matter in 90%+ of motors but everyone like to think it does.