oil weight

Mercruiser recommends their branded 25w40 across the board in the inboard and sterndrive engines. That is what I have always used. You can't go wrong with that, but there are also other brands that are fine, problem is the 25w40 weight is not common. If you have or can find your manual it recommends some alternatives if the 25w40 is not available. There are lots of debates on oil, my opinion is with today's oils, about any good brand 10w40 is fine. I figure Mercruiser recommends the 25w40 because - well - they sell it! These are GM blocks, nothing special there. Volvo recommends a 10w40 synthetic in their GM based engines FWIW.
 
Have done testing in my 350 mag and 496. Any major manufacturer 15w40 oil works perfectly.
Readily available for less than $15 for a 4 quart jug.
I suggest Rotella or Delvac as my preference of them.
 
The main difference between boats and cars is that boat engines run at high load virtually all their life keeping many tons of boat on plane. Cars run at much less loads. You need a good high viscosity oil to make your engine last. Oil is the cheapest part your boat uses. Oil change is less than 1/2 tank of fuel. Price difference from the OEM recommended to the best auto 10/40 is like 1/4 tank of fuel. Don’t cheap out on it.

Not for cost reasons, I am wondering whether diesel rotella 40w is better than Mercruiser oil though.
 
Creekwood said:
Not for cost reasons, I am wondering whether diesel rotella 40w is better than Mercruiser oil though.
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"Better" is subjective. Merc 25w40 Catalyst compatible oil is not a very stout oil when it comes to additives. In order to be catalyst compatible the high pressure boundary layer additives need scaled back to avoid potential catalyst poisoning. As a result, there are oils for older boats without catalysts that contain more additives and could perform better. Most diesel oils fall into this category.
 
paulswagelock said:
"Better" is subjective. Merc 25w40 Catalyst compatible oil is not a very stout oil when it comes to additives. In order to be catalyst compatible the high pressure boundary layer additives need scaled back to avoid potential catalyst poisoning. As a result, there are oils for older boats without catalysts that contain more additives and could perform better. Most diesel oils fall into this category.
Click to expand...

That is interesting. So the newer Marine oils may not actually be that great for those of us with the pre-catalyst hunks of iron in our bilges? The marina does my changes in the fall and uses Mercruiser Semi-synth 25w40. Maybe time to switch to Rotella next change for me.
 
I've been using Sierra 25w40 in my 8.1s. The 5qt jug is more economical than the Mercruiser stuff, and I'm quite sure neither Mercruiser or Sierra refine their own oil, so someone else is making it for them to badge, and I'd bet it's the same source for both.

In my previouse boat, with 1989 Gen IV 7.4s, I used Delo 400 15w40 for about 8 years. Never had any oil related issues.
 
So the catalyst compatible oils are similar in additive chemistry to the newest car engine oils when it comes to antiwear chemistry. That is not necessarily bad as they sometimes use different base stocks or other replacement additives like boron or moly.
Key things you want in a boat oil are sufficient antiwear additive, stable viscosity, high hths and low Noack. That is exactly what a good diesel oil has.

The Merc syn blend is a fine oil and will work on the older engines. My point is that it likely is not “better”.
 
I used Rotella 15W 40 for years in my old boat with the original motors and never had a problem.
I switched to the Mercruiser 25W 40 when I repowered to keep wishing the warranty and wound up sticking with it.
 
Royal Purple HPS 10w-40 with Royal Purple filters (they seem to be much stouter than any others I've seen). (4.3's, 5.0's with a couple of cylinders missing....). They used to do a Marine spec'ed oil, but their engineering folks recommended this as better when I spoke with them.
 
15/40 here all season long. Live further north than most of you though. 15W sounds better than 25 during cold weather start ups and for a fraction of the price. Both 25/40 and 15/40 are at the same viscosity before it goes into gear.
 
Penrite Racing 15/50 here. Poa/ester oil with no viscosity modifiers and heavy dose of zinc/phos and boron with decent HTHS and TBN. I was getting some consumption with the 25/40 merc oil after certain hours of use (but within normal consumption per manual). The Penrite oil stopped consumption in its tracks.
 
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