lawn mower wont start. oil has gas in it?

Well I'm guessing the gas in the oil came from using the primer pump over & over again. I doubt that's the cause of your problem. Have you checked compression yet? The guages are $40 at the parts store & you should have at least 30psi. My generator would run as low as 40psi but I knew that was really low so I had the valves lapped in & got it up to 90. If your compression is low then take the head off & see if you can see the rings around the piston. I was told by the mechanic at the lawn mower shop that if you can see 1/8" of the rings then it's toast. However, I'm betting if there's that much cylinder wear, it would have quit running long ago. I doubt you should be able to see any rings at all.

There's also another possibility that I discovered. Sometimes the emergency cut off cable that runs from the handle & you hold down to keep it running will stretch. When that happens it keeps it grounded & it won't start. All you can do is buy a new cable or drill a hole about a 1/4" further & run the cable into that. That's what I did.