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My boat has the the original engines that came with it in 1978. The Mercruiser owners manual specifies 93 octane gasoline . I bought the boat from the owner's estate so I can't very well ask him. The guy who showed me the boat was putting 87 octane in her and the guy at Oakdale Yacht gas dock said that should be fine. What are others using in these engines?

Also, the Mercruiser owner's manual says to use straight weight motor oil. Is that still valid or should I be using multigrade?

Thanks,

Hi Peter,

Others here know much more than I, but I'm just popping in the let you know (only if you already haven't be told) that ethanol in the gas will dissolve nasties from an original fuel tank and deposit crap in your engines...I'm sure someone else can give you more info, I just didn't want you to go ahead and top her off (I ran diesels)

Thanks for the heads up.

I know about the ethanol issue. The boat has an aluminum fuel tank.

Given that she is in the yard and we have a foot of snow on the ground here on Long Island it will be a few months before I top up the fuel tank.

I had older mercruiser 454s in my B31. I used regular gas, and mercruiser oil. You'll be fine with regular. Great motors.

Oakdale-- that's Bobs Licos marina if I recall correctly.


I could be wrong, but have understood if using less then recommended octane you need to be sure motor does not knock. If it does either add octane boost or retard timing a hair till knock subsides. No knock, no problem.

Also around here, it's 89 octane at the pumps as a minimum, but usually premium 91.

Peter

If I remember correctly, my 454's which were 350 HP each, called for 93 Octane also. After a point we could not get that level of Octane at the Marina's, you can get close at the gas station if you want to lug it.

We ran the 89 for years with no problem and we repowered in 2008.

Originally we ran mercruiser 30 weight as specified. When that became scarce we used Mercruiser Multi Viscosity. No issues. My father was a mechanic, in fact, we broke another rule with our engines. Due to the pitch in the engine, my father was afraid of the front half starving for oil when running so he used to run an extra quart in her. Never had an issue. Never foamed on us.

We used to change the oil every fall. We would buy cheap 30 weight or multi for the fall lay up, and left the filter in place. In the spring we changed it to the Mercruiser oil and changed the filter at that point.

When yo pulled the valve covers on those engines you would not find a spec of sludge. Not a piece of dirt.

Been using straight grade 40 wt Rotella in my vintage 80’s 454s for the last 14 seasons. Approx 1400 hrs on one engine, 1200 on the other. I use whatever regular gas there is available at the dock, which is usually 87 or 89 oct.

I use Shell Rotella T 15W/40 in my 1989 Crusader 454s.
Local gas on Narragansett bay is usually 89 octane with ethanol. I add Sta-Bil marine to make myself feel better.
Good luck-
Stephan

Peter

I think the as you can see, use a good oil and don't worry about the gas unless you hear those engines ping. Mine never pinged even when I cranked them up.

Older gas engines remember used the lead in the gas as a lubricant for among other things the valve guides. Many times if the heads have been pulled for repair, guides were upgraded.
Customers who have the old original engines in the muscle cars we service are advised to use relead in the tank.
Many Marina gas tanks carry non ethanol fuel with valve tech and other additives already.

We use a straight grade of oil down here in the warm to hot weather, 40w mostly.
10w40 would be okay in colder climates both in air and water temp.

If you experience ping, you can back off timing if you want to keep the cost of fuel down to a lower grade as the case with gas or diesel running the engine out to the end is not a regular occurrence.

Rotella T 15W-40 and 87 pump gasoline with Ethanol as dictated by NYS availability. No problems other than having to change fuel filters several times when Ethanol was introduced.