A mechanic recently told me that the rear main seal on my 1980 450sl needs to be replaced soon to keep the oil pressure going to the cam shafts from falling. The pressure gauge indicates "3" when driving and between "1" and "2" during idle. Since I see no oil drips on the driveway, is this mechanic pulling my leg and looking for an extra grand, or could this be more than a potential nuisance problem? Thanks