Be sure to check the oil level! If it still looks and feels like oil and is at proper level you can probably safely fire it.
If oil level is high there may be water in the crankcase. If it's low or doesn't look, feel, or smell like oil then you better change the oil before firing the engine. My opinion...
If you put a gallon of Marvel Mystery oil in the cylinders then you better change the oil before firing! I would expect MMO to run down the cylinders and into the crankcase. Why not just use motor oil in the cylinders if you feel that's needed?
