Gas in the oil

Gas is in the oil on a 2017 Yamaha outboard . How is this happening and what has to be done to fix it. Thanks.

Kind of a late entry, but on 4 strokes it's common for the diaphragm on the external fuel pump to rupture putting gas directly into the powerhead (upper end). The gas then leaks down into cylinders and into the oil. I have a pump on order, and expect this to fix my problem. I had already changed the thermostat, which was stuck open. I changed my oil this Spring, took one fishing trip, and ended up with so much oil it was at the top of the dipstick hole. That can only be one thing causing that. Once I remove the old one and install the new one I'll post pics of both.

What oil are you running in it.  I had the same issue on a mercury with a yammi powerhead ran mobile one in it one season and ended up with gas in the oil went back yo nmma certified oil and problem solved. 

Gas passing between the cylinder walls and piston rings is the only place this mixing can take place. The listed causes are all possible. Bad temp sensor/thermostat causes computer to run rich too long flooding cylinders. Long term low idling can do the same thing. Worst case possible is stuck or bad rings. This is not a problem to put off for another day. All of these causes will destroy your engine fairly fast. 

All of the possible causes need to be eliminated and I would start with the worst case first. I am a do it yourself kind of guy.  First step is compression test. If you are a diy guy, the tester is cheap and I am sure you can YouTube the procedure. If not find a good marina that won't bend you over the port bow.

Makes sense. 

Plus one on the rich mixture caused by thermostat open. Unburned fuel washed down the cylinder walls and contaminates the oil.  

Normally this is caused by leakage between piston rings and cylinder walls. JMO Using the wrong oil(not recommended), could be part of problem but Rings are also the culprit. 

Not an atypical issue.  a lot of idling at slow speed can contribute to this as well.   Change oil, and run it good and hard.  You may see this also posted as "making oil"  At low speed the excess fuel can be bled into the crankcase.  If I am going to do a lot of slow speed trolling I use the kicker.  Other wise I don't put-put along with the 115 Yamaha very much and that seemed to solve my issue.  

I had this happen on a 2016 Honda 40 when my uncle used spare automotive oil instead of marine. He didn't think it would make a difference but the engine did NOT agree. Fuel slowly ended up the in crankcase after every use until I noticed it and the dipstick smelled like gas and the oil was very thin. Switched back to Honda factory marine oil and the problem stopped. May not be your solution but something to consider. 

First thing to check is your thermostat. A thermostat stuck open will cause your engine to run cold and rich.