I have recently acquired a 5 horse mercury freshwater, outboard, transum hung engine.
What Im not sure of is do I mix the gas with oil or not. Thanks

Take the cover off and look to see if it has valves. Valve cover is a dead give away.

A 4 cycle has valves. A 2 cycle does not.

The 2 cycle will have compression on every stroke. The 4 cycle every other stroke.

You could also go to the mercury website and look up your engine. There should be some kind of id plate on the motor.

If that deosn't work there are a few other ways to tell which one you have.

EJ