My Kobalt brand compressor is about 3 years old. A few months ago it began leaking oil (mineral) from the pump base and drain plug & valve. These were wiped clean, retightened and oil added to specs. given in the owners manual. It took about 7 ounces to fill. Since then this process has been repeated. It seems odd that oil would get down into the tank. Bad seal(?) Any ideas???

Also why is mineral oil specified instead of light weight motor or synthetic oil?