I have a 95 Mariner 200 EFI that I recently took off the oil injection and now mix gas and oil. Since doing this about 2 months ago I have only put Mercury oil in the tank and it smokes really bad and Im on my 3rd set of spark plugs. What oil can I mix with that will burn cleaner with less smoke and longer spark plug life?

What ratio are you using to get all that smoke and plug fowling?

50:1

Something doesn't sound right... With that mixture I wouldn't think you would see that much smoke or fouled plugs that quickly. How are you mixing the oil? Are you pre-mixing it before putting it in the tank or adding gas then oil?

Mercury now recommends DFI oil for less buildup, if you can premix the gas before you add it in as Sfritr suggest. Also you may want to try sea foam at the spike dosage for a couple of tank full which I think is about 2 oz./gallon, but check the back of the bottle. I use Amsoil HP Injector with very good success runs CLEAN and have had luck with Pennzoil Synthetic on my smaller outboards.

Good luck.

Do you spend alot of time idiling? That would also cause an issue.

No I dont idle much at all. I usually pour in the oil, lets say 1/4 gallon and then put in Quickleen, then stabil, then I put in the gas. Ive heard the Mercury oil smokes alot and Ive seen it with my engine.

Sounds like oil is not the root cause of your problem. If you are going through plugs that fast, smoking badly and hardly ever idle, it's time for a serious tune-up, starting with a good linc and sinc, the the carbs and possibly new jets. If the motor has many hours, it's possible the jets could be worn. Yes, jets do wear as the mixture goes through them.

As for oil, Penzoil 100% synthetic is a good oil for carburated motors.