4 stroke outboard oil

[Gerrycooper56]

I've just bought a new Tohatsu 15hp 4 stroke outboard and wanted to use the same oil I use in the genset and main engine. The dealer and manual said only use mineral oil and I'm wondering why. The dealer said it was about heat and heavy duty but synthetic oil is used a extreme duties like motor racing.
Any thoughts from the experts?

[ontherocks83]

I am assuming the main engine and gen set are diesel and the outboard is gas. If so then no do not use one for the other.

More details as to what each engine calls for would be helpful if they are all the same fuel type

[Gerrycooper56]

The genset and main are diesel with 5w 40 synthetic. Apart from TBN I would have thought there is little difference between a diesel and gas engine oil.
The outboard says 30 grade so 5W 40 should be ok for viscosity.

[ontherocks83]

It may be "doable" to be honest I'm not 100% sure. Coming from an aircraft maintenance background I cringe when ever the wrong part is used for a job unless its an emergency.

I know space on your boat may be at a premium but honestly whats a quart or two. Think about how much you just spent on your brand new engine. Do you really want to risk it over some space, convenience, and a few dollars.

Maybe someone else has some more technical info but remember the manufacturer put those specs on there for a very good reason.

[a64pilot]

A good grade of Diesel oil exceeds spark ignition requirements by a wide margin, but it is not an energy conserving oil. Many sport bike riders have discovered this and use Rotella and Delo oils.
Gerry knows his stuff, but check with the folks at www.bobistheoilguy.com

[Gerrycooper56]

Nothing to do with space or convenience really. I want to use synthetic precisely because of the cost of the outboard. My experience with auto engines is that synthetic is superior to mineral and the extra price is worth it. You're right of course that the manufacturer specified mineral for a reason.

[Stu Jackson]

Have you read the labels on the oil cans (plastic jugs)? They'll tell you better what the service is for, better than any of us here. Good advice that some engines asre diesel and the new one is gas.

[Gerrycooper56]

Stu,
Yes I have read the labels but it generally refers to a specification