I think the troybilt 140cc walk behind 21in 3 in 1 mower takes 15/30? I have not started or run the lawn mover yet-and there is a separate area for filling up gas and filling up engine oil. Maybe this engine is a 4 stroke and not 2 stroke? Was doing a favor for a friend but think I messed up. If this is the wrong oil-how do I remove it from the mower before finishing the lawn?

Drain it out, look for plug under deck. Most mowers use straight 30 wt. It is a 4 stroke.

Hi,
The thing to do is check the engine oil level. Most hold 16 - 21 oz. Make sure it is to the full mark. Engine is a 4 stroke. It will run on the mised fuel just fine. Just put in gas nest time.
Peace,
Clarke

Now I'm a little confused. Did you put the 2 stroke oil in the crankcase? If so, how much? Or are we talking about putting straight oil into the gas tank?