Whats the best octane rated fuel for two stroke outboards.

Hi all, Ive got a 2003 125hp Merc (oil-injected 2-stroke) on the boat. The mechanic that fitted it said its best to run 95 octane unleaded. Motor runs fine on that and not really very smokey. Is it even better to run 98 octane through it permanently or could that cause any internal damage, overheating etc cheers dave

Posts: 1009

Date Joined: 19/06/14

Doesn’t matter

Wed, 2020-01-08 16:04

Two bangers have been around for eons running on whatever you feed em . If you're feeling frivolous and want to give her a drink of high octane go for it ! 


 My wife understands why I clean my rods n reels in the shower....


Posts: 50

Date Joined: 01/11/16

The only thing 98 will do is

Thu, 2020-01-09 08:53

The only thing 98 will do is make it run a little better than 95. You'll have no issues running it on 91 either. I fill up with 91 but if the boats not been in the water for a couple of months I get 98 just to lift the octane rating of the older fuel. 

JohnF's picture

Posts: 2832

Date Joined: 07/07/10

 With low compression 2

Thu, 2020-01-09 09:19

 With low compression 2 stroke, not sure 98 will do Anything positive at all. 91 is all I would use


I use 98 only on my Verado, but it is supercharged and chipped for higher boost, totally different story than a stock 2 stroke  


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Date Joined: 08/09/11

 Not sure about synthetic

Thu, 2020-01-09 20:23

 Not sure about synthetic oil, but mineral oils lower the octane rating of petrol, 

The only problem with high octane fuel is it requires more spark advance. The theory back in my racing days went- using high octane fuel without advanceing the spark was running with retarded spark, causing the motor to run hotter -  .

I don't think an outboard motor would have any problem, maybe not be as fuel efficient, but doubt it will be noticed. 

I admit I put 98 in with stale fuel, and have never had an issue.