Two-stroke 15-horsepower Mercury outboard motors require a 50:1 fuel-to-oil mixture. However, not all 15-horsepower Mercury outboards are two-stroke engines; many newer models are four-stroke designs. Four-stroke engines have a lubrication system similar to a car, with a separate oil reserve, and don't use pre-mixed fuels.. Other models require the user to mix the oil and fuel together in the outboard's main fuel tank each time the fuel tank is topped up (this is known as a premix model) In addition to this you will need to know if your outboard's oil/fuel mixture is either 50:1 or 100:1 so that the correct amount of oil is poured into the fuel tank.
The correct fuel / oil mix is 50:1. This would be a pint of oil to 6 gallons of gas or a quart of oil to 12 gallons of gas. If you can, pick up a ratio container for those times when you have an odd amount to mix. Also make sure you use a good quality outboard oil with a TCW3 rating.. Mercury Outboard Fuel Mixture Chart. If you can't find the expected performance from your Mercury outboard motor, it can happen that the fuel mixture ratio is not appropriate. And this issue is the most customary among people concerned about the Mercury outboard. Putting too much oil into your Mercury outboard motor will be worse.