1994 Chevy silverado 6.5L turbo diesel: My oil pressure is between 40-60 for the first few miles on the road. then drops and seems to hold steady at around 40 while my foot is on the gas, then drops between 20-30 when stopped. check engine light flashes periodically, but not all the time. i noticed my oil filler cap might be faulty as there seems to be some seepage, but im not loosing much oil. This all started after i had my oil changed (standard) 15w40. and it was slightly overfilled. is this a cause for concern or should i just replace the filler cap and get another oil change? thanks

I'm not sure why you want to change the oil again. Your oil pressure numbers appear to be normal. Replace the oil cap if it's bad. You'll have to scan the ECM for codes to find out why the the check engine light comes on.

If you used standard engine oil (petrol oil ) then it will not have the essential ingredients need for the higher temperature of the turbo and will soot up easily and will not have the tensile strength of an oil needed for a diesel . It is not the viscosity of the oil that counts but the additives. In short you can use diesel oil in a petrol engine and it will prevent sludge build up but you cannot use petrol oil in a diesel unless you want early engine failure.. As for the pressures they appear to be about right. The engine light may be flashing because the crankcase system is not being controlled for pollution requirements so fit a new cap and go from there.