I Put An Oil/gas Mixture In The Oil Tank Of My Lawnmower What Should I Do?

Tip the lawn mower over on its side and try to drain out as much of the mixture as you can. Not sure what would happen if you did let the engine run with gas in the oil reserve? Good luck:)

Drain the oil and refill with the proper oil.  Most push mowers have a drain plug underneath the deck on the bottom of the engine. Most riding mowers have a drain plug on the side of the engine block right above where the engine meets the frame.  Usually a messy process so keep plenty of rags handy.  If this is confusing someone at your local hardware store in the lawnmower section can probably point you in the right direction and point  the drain plug out on one of their new ones on display.