mikestrobe201
Active member
Getting ready to change the oil in my Mercury 350 Magnum in preparation for running for the first time in 18 months and wanted some input on what oil to use. Should I go with the Mercruiser 25W40? 
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Best engine oil for Mercury 350 Magnum?
- Thread starter mikestrobe201
- Start date
Chris E
Well-known member
that's a fine oil, any name brand straight weight oil will suffice - SAE 30 or 40. we used to use penzoil SAE 40 at one marina i was at. My own boat (86 Sea Ray w/3.0L)gets Amsoil 15W40 that engine works hard, so i don't mind giving it a treat. the merc oil is a mix of 25 and 40 weights, so its actually a 33 or something. do not use mutli grades in a marine engine (10w30) they froth up under the higher loads that a marine engine endures. the only exception to that rule is the amsoil as it is a straight 40 that has such good flow at low temps that they can add the 15 to it. they say that load a marine engine is under is the same load as running your car engine up a mountain all day pulling a trailer.
SneakyPete
Member
I used to use Shell Rotella diesel oil, 15/40, now I used Mobil 1 20/50 synthetic Extended Performance Gold cap. I have never had multi weight oil "froth up" in my boat, nor have I ever heard of a multi weight oil "frothing up", nor can I understand how multi-weight oil would "froth up." In my experience, oil froths up only when mixed with water/coolant, or when the oiling system is not set up properly or is over filled, and the crank aerates the oil. If I were you, I would use a good quality synthetic oil, in the viscosity recomended in the manual. A bunch of monster motor guys I know run motorcycle/V twin synthetic oil in their motors due to the increased anti wear additive package. That is why I was running the Rotella when I had a flat tappet cam, becuase it had more Zinc and ZDDP.
81 enchanter
Active member
i second the rotella, i also use it in my bike
winter
Active member
I 3rd the Rotella. As Sneaky said, it has the most zinc/zzdp additive which the feds are trying to lower. Shell gets away with it bc the bulk of their sales are to big truckers who i beleive as exempt from 'oil rules' due to the miles they drive between oil changes.
I'd use their 15/40 product that can be bought by the gallon at walmart. I actually founf Autozone carries the 5 gallon buckets of the stuff for cheap.
Frothing of oil is not normal for the reasons mentioned here. However, oil that has been overheated at high RPMs can show signs of frothing/foaming. You def. want to add a bit of viscosity in exchange for engine drag, in the name of engine longevity. My engine builder built me a 540ci 740hp for my street car and told me to use nothing but Pennzoil 20-50 racing, a conventional oil with specific properties for high load applications. I don't know how much of that is true, i only cruise my Camaro, not race.
Anyway, just make sure you use a very good filter. I pref. Mobil-1 filters as they have synthetic media instead of paper/cardboard. If they are hard to find, my next step is any German oil filter.... Mann, Knecht, Mahle, Bosch. Stay away from Fram unless it's the HP series.
I'd use their 15/40 product that can be bought by the gallon at walmart. I actually founf Autozone carries the 5 gallon buckets of the stuff for cheap.
Frothing of oil is not normal for the reasons mentioned here. However, oil that has been overheated at high RPMs can show signs of frothing/foaming. You def. want to add a bit of viscosity in exchange for engine drag, in the name of engine longevity. My engine builder built me a 540ci 740hp for my street car and told me to use nothing but Pennzoil 20-50 racing, a conventional oil with specific properties for high load applications. I don't know how much of that is true, i only cruise my Camaro, not race.
Anyway, just make sure you use a very good filter. I pref. Mobil-1 filters as they have synthetic media instead of paper/cardboard. If they are hard to find, my next step is any German oil filter.... Mann, Knecht, Mahle, Bosch. Stay away from Fram unless it's the HP series.