What oil should I use?

DanOnTheBoat

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Sep 15, 2024
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So... looking to change my boat engine oil this year before winterizing it. What oil should I use? I have a 4 cylinder 2.5, 120hp merc inboard/outboard engine. Its an 87. It has an oil cap on it saying 10W-30. I looked online to see what comes up, I found few online a Quicksilver to be specific its a 10W-30 the thing is it says that its for us in 4-cycle outboard engines. Would I be able to use that in my engine? It seems like all of these oils say outboard and none for inboard. Although, there is a 25W-40 oil that is for us in outboards and inboard engines(still Quicksilver) Would there be a problem if I use that oil in my engine? Where would i buy my oil from to make sure its compatible with my I/O?(Canada) Also I heard that full synthetic isnt really good for cast iron blocks, is that true?
 

Bondo

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Ayuh,..... I run Rotella 15W-40 in everything I've got,.....
You can run any oil you'd like,..... just don't over-think it,.....
 

Lou C

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Just not an automotive 5/30 that will thin out with sustained high rpm use. 15/40 or 20/50 are good and much cheaper than the marine oils.
 

DanOnTheBoat

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Sounds good Ill go with the 25W-40 that I found online. Rated for inboards and outboards.

Would any harm be done if I run regular full synthetic car oil in my inboard? Or are they different oils?
 

JASinIL2006

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Full synthetic will be fine. I run it in my boat engine. Honestly, any oil will be fine as long as you use the right viscosity and change it after every season.
 

paulswagelock

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Keep it simple. Run Delvac, Delo or Rotella 15w40 in it. if you want, use their semi synthetic blend versions but not even necessary. These 3 are available everywhere and inexpensive.
 

Bondo

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Rotella makes marine oils? Or are all of those brands for cars? Found Rotella 15w-40 T5 oil. Would that work for a 87 I/O 120hp merc motor?
Ayuh,...... As it says right on yer oil cap,...... Engine oil,....
Rotella makes fantastic Engine oils,.... it'll work perfectly for yer motor,.....
 

Lou C

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that label is a GM label for an automotive application no doubt
Merc never recommended an oil of that viscosity in their inboards (they recommended straight 30, 40, 20/40, 20/50 or their 25/40).
if you don't want to use high priced "marine" oils another option is
Valvoline VR-1 20/50, this probably has even more zinc than Rotella or Delo, by a large margin. Also cheaper than the "marine" oils as well.
 

Scott06

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Use the T4 conventional rotella . It has enough zinc for flat tapped cams And is an excellent value
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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dont overthink it. you need zinc in the flat tappet motor you have.

as stated. get the rotella T4 15w-40 diesel motor oil.
 

airshot

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Very very few engine issues are oil related, most are from lack of or improper maintenence !!
 

jimmbo

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So... looking to change my boat engine oil this year before winterizing it. What oil should I use? I have a 4 cylinder 2.5, 120hp merc inboard/outboard engine. Its an 87. It has an oil cap on it saying 10W-30. I looked online to see what comes up, I found few online a Quicksilver to be specific its a 10W-30 the thing is it says that its for us in 4-cycle outboard engines.Would I be able to use that in my engine? It seems like all of these oils say o utboard and none for inboard. Although, there is a 25W-40 oil that is for us in outboards and inboard engines(still Quicksilver) Would there be a problem if I use that oil in my engine? Where would i buy my oil from to make sure its compatible with my I/O?(Canada) Also I heard that full synthetic isnt really good for cast iron blocks, is that true?
Mercury/Quicksilver offer more than one 10-30 Oil, some are labelled for Outboards, some aren't. The Engine Manufacturers know their Engines and what they require to avoid a Horrible Twitching Death. I know there are some here that will insist that there is No Difference between a Auto/Truck Engine and a Marine Engine... Well a Marine Engine works-runs under an entirely difference set of Parameters, and thereby subject to different Demands, and that can place different demands on Lubricants, and might need a different Formulation/Additive Package more suited to the Marine Operational Environment. But to some Oil is Oil...
 

KJM

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I would just use whats called for on the cap. 10w30 regular engine oil.
 

Lou C

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That’s there because the oil cap is put there by GM for automotive applications but that’s not good for marine use running at 3500-4000 rpm all day….
Merc says 25/40 and you can use 15/40 or 20/50.
 

Scott06

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That’s there because the oil cap is put there by GM for automotive applications but that’s not good for marine use running at 3500-4000 rpm all day….
Merc says 25/40 and you can use 15/40 or 20/50.
Exactly my similar vintage 3.0 had same cap on valve cover but merc documentation recommended straight 30 w or 40 w depending on ambient temps
 
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