I'm assuming this is Sta-bil, seems to be the most popular brand. It won't hurt anything. I would try a quarter tank in the mower to see if it runs ok, if yes fill up the rest of the way, if no then add straight gas to dilute. One or 2 oz per gal I believe is the recommended rate so that would be 8% tops. Mower may run funky but no damage.

Shelf life on stabalizer is about 2 years.

Pour some of the mixture into your car tank and then refill your mower can..

I had someone a few years back that added stabil like you would 2 cycle oil until the carb was pink inside overtime. Finally, the carb plugged and I couldn't save it. I'd take popcornhill's advise myself. Put it in the proper amount of gas so no damage is done to the car.