To the Doctor:
I have an engine with over 100,000 miles. I’ve read that full synthetic oil is the best protection for an engine and that high mileage oil is best for engines over 75,000 miles because it helps condition seals.

Which oil should I use for my engine?

The Doc Says:
If your car’s engine is in good condition, meaning it has no visible leaks and doesn’t burn a quart of oil every month, and you truly want the ultimate in protection for it, then synthetic motor oil is the way to go. Synthetic oil offers superior cold and hot temperature protection, and it is much less likely to “burn off” and form sludge when it hits hot spots in the engine.

However, if your car is consuming a bit of oil between oil changes, it might be worth it to switch to high-mileage motor oil. While high-mileage motor oil is usually formulated from conventional petroleum base oil, most high-mileage motor oil formulations have a specially blended additives package that can help to preserve and protect the seals inside your engine. Plus, as it is a little higher in viscosity than conventional or synthetic oil, it is less likely to squeeze between gaps between the piston rings and cylinders in your engine, where it could burn off during combustion.

So the long and short of it, in the opinion of this “doc”, is to go with synthetic if your engine has been well maintained and is in sound condition, and go with higher-mileage motor oil if you are experiencing a bit of oil consumption between oil changes.

A Second Opinion:

Synthetic oil should be continued in vehicles that require synthetic oil for service fill. Synthetics offer outstanding low-temperature flow, oxidation protection and deposit prevention and are an excellent choice for older or newer vehicles.

For vehicles that are showing minor oil seepage, a little more oil consumption, a little “blue smoke” at startup, slight performance reduction or increased engine noise, high mileage oils are often the best choice for vehicles that allow use of conventional oils. These are some of the issues that high mileage oils can address effectively.

Steven Balfe
Technical Advisor, Shell Lubricants

The advice described above is for informational purposes only. It cannot and should not be used in lieu of an actual, physical inspection and diagnosis by a trained mechanic or automotive technician. The opinions and advice offered herein are not intended to diagnose automotive problems or component failures; they are simply intended to provide information on what could be transpiring. VehicleMD accepts no liability resulting from actions taken as a result of this advice.