How much should my time be worth?

About every other night, my Dad makes me run down to the gas station and fill a 8 gallon tank up with Kerosene to fuel my house with heat.(He didn’t pay the oil company for the previous $600 bill, so he makes me fill it with this…which I don’t find very safe at all) He pays me $20 to fill the tank up, and thats about it… Details below…

(Carrying a 8 gallon tank for about a 2 minunte walk, along with the weather.[Tonight it was hailing out, and i was freezing my ass off] Plus gas back and forth from my house to the station, thats about 3 miles. Some nights its doubled too…)

So what do you think i should do? I was going to maybe change him a fee every time he wants me to do it, but i don’t know how much…