Can You Switch an Older Car to Synthetic Oil?

The question comes up at our service centers most weeks, usually from someone with a 2006 pickup or a 180,000-mile sedan who heard years ago that synthetic oil would make the engine leak. Synthetic oil has changed a lot since that rumor started, and so has the testing behind it. Here is what matters when an older engine moves to synthetic, and the few things worth watching afterward.

At a glance

• Synthetic and conventional oil of the same grade mix safely. You can switch either way.
• The leak myth traces to early synthetics decades ago, not current oils.
• An engine that already seeps may show it more after a cleaning oil goes in.
• High-mileage formulas add seal conditioners and come in synthetic versions.

Is it safe to switch an older engine to synthetic?

It is. The American Petroleum Institute licenses oils by grade and service category, not by base stock, so an API SP 5W-30 synthetic meets the same requirements as an API SP 5W-30 conventional and then some. Synthetic and conventional oils of the same grade mix without trouble, which is why a synthetic blend exists at all.

AAA's 2017 testing found synthetic oils outperformed conventional oil by an average of nearly 50 percent across industry-standard tests, and the benefit shows up most in heat and cold. That matters in a place where the same car sees 100 F parking lots in July and single-digit mornings in the Carson Valley in January.

Where did the leak story come from?

Some very early synthetics did not play well with the seal materials of their day, and that reputation stuck around long after the formulas changed. Today the more common situation is an engine that already has a weak gasket or a hardened seal, held shut by sludge or varnish. A strong detergent oil can clean that deposit away, and a seep that was always there becomes visible. The oil did not cause the leak. It revealed it.

That is worth knowing before you switch a very high-mileage engine. It is not a reason to avoid synthetic. It is a reason to look under the car for a few weeks afterward.

Should it be high-mileage synthetic instead?

For many engines past 75,000 miles, a high-mileage oil makes sense. These oils add seal conditioners that help older rubber seals stay soft, and most brands offer full synthetic high-mileage versions. If your engine burns a little oil or spots the driveway, the high-mileage oil change is often the better fit. If it is dry and quiet, a standard full synthetic is fine.

1. Confirm the grade in the owner's manual or on the oil cap.
2. Pick an oil that carries that grade and the API category or maker specification listed.
3. Choose high-mileage synthetic if the engine is past 75,000 miles and uses or seeps oil.
4. Check the dipstick and the driveway a few times over the next month.
5. Keep the interval your manual lists. Synthetic does not let you ignore it.

Does switching to synthetic change the interval?

Only if the manual says so. Some manufacturers give a longer interval for synthetic, and AAA puts many modern engines at 5,000 to 7,500 miles on it, but an older engine with a published 5,000-mile schedule keeps that schedule. The added protection is a margin for hot summers, cold starts and the occasional long gap. It is not a license to double the miles.

What we do about it in the bay

When an older car rolls into Longley Lane, Mae Anne Avenue, College Parkway or US 395, the technician checks the grade, looks at the engine for existing seepage while the car is up, and tells you what was seen before you pick an oil. If the valve cover or oil pan is already damp, we mention it so a switch to synthetic does not surprise you a month later. We do not repair gaskets, so if a leak needs work we say that too.

You decide between full synthetic, synthetic blend or high-mileage. No appointment needed, and the next due mileage comes from your manual.