First and foremost the gas station owner is an idiot, anyone doing this type of crap in this day in age with Yelp and other online service reviews must have an IQ of 60. Eventually the word will get out that this guy is a crook and he'll have no business.
The best approach is to find another job, you don't want to work for a guy like this. He cheats his customers he will also cheat you. And worse yet eventually you'll have to cheat customers which sells your soul. The best approach is to call anonymously to ExxonMobil here's the customer care number:
http://corporate.exxonmobil.com/en/company/contact-us/directory/service-station-customerrelated
I assumed it's an Exxon gas station since you mentioned Mobil 1 oil container and Mobil 1 is usually only available at Exxon (ExxonMobil) gas stations.
Good luck... Remember a coward dies a thousand deaths a hero dies but once, good for you for reckonizing a crook who is his cheating customers, and that he needs to be reported, After-all what happens if it continues his bad deeds and somebody gets in a bad auto wreck because brakes pads weren't replaced only painted over.