Synthetic oil in a 4 stoke outboard

[peterbud]

I have a honda 9.9 four stroke. I was wondering if there is a anadvantage or disadvantage to using synthetic oil.

 

[Maine Sail]

I don't see why

I have a honda 9.9 four stroke. I was wondering if there is a anadvantage or disadvantage to using synthetic oil.

No reason you can't, but why? After break in it's fine but over kill IMHO. Auto & truck engines will run for hundreds of thousands of miles these days on just conventional motor oil. Personally I would opt for more frequent changes over going synthetic. On older engines there are lots of us who have experienced seal leaks when converting to synthetics including myself.

Our diesel has 2878 hours and has only ever used conventional oil. That's nearly 29 years worth of motoring for the average sailor @ 100 hours per season!

Valve cover as it came off the engine @2878 hours:

The top end:

Hours: