You haven't shown all of your code so I will assume that lubeSubTotal, oilChangeCost, and lubeJobCost are global variables. All you need to do is in your ClearOilLube() method set each varible to 0.
lubeSubTotal = 0;
oilChangeCost = 0;
lubeJobCost = 0;
or, you can actually do it in 1 step, which I don't prefer because of readability:
lubeSubTotal = oilChangeCost = lubeJobCost = 0;
It's read right to left. The assignment of 0 goes into lubeJobCost and that is assigned to oilChangeCost, etc.