You do not say the make and model of your mower.
On my really cheap petrol mower there's a separate, capped-hole for engine oil - that's a bit of a clue - so I put oil in the oil hole and gas in the petrol tank.
On my miniature garden tiller there's no hole for oil - it's obviously a two-stroke so I mix two-stroke oil with gas and pour that mixture into the gas tank. Mixtures tend to vary from about 50:1 to about 25:1 depending...
Peccavi, May 2010
