Frame needs oiling...?

I have another thread regarding the transmission, but, it appears my clutch is gone. So, it is going to be sitting for a few months til I can replace it. In the meantime, I am going to do other repairs. Last year, when I had it inspected, the mechanic beat the underneath of it with a metal pole, and, yes, pieces of rust fell. He told me if I did not get a coat of oil under it, it would not pass again. It has 115,000 miles, and, living in Maine, all of our cars get chewed up by the road salt. My question is...how do you oil it and what do you use??