maybe try changing the oil to a higher viscosity or simply change the oil. My 1990 300 was registering no oil pressure after warming up yet still ran, the dick that I bought it from had put synthetic in. I changed it to Castrol 20W-50 and the pressure is right on the money. I feel lucky that the damn thing doesn't leak through seals. I swear that these people that try to 'upgrade' routine maintenance via new products in fact wreck these things. Don't try to do this. Just put it back to what the factory said to do in the first place and hope for the best.