Can you use synthetic oil in an older car?

Yes. Synthetic oil won't hurt an older engine, won't make it leak, and in most cases will protect it better than the conventional oil it's been running on. Porsche has factory-filled with synthetic since 1996, and nothing about a 1998 Civic or a 2006 Silverado makes synthetic a bad idea.

The one real caveat is zinc. If the engine has a flat-tappet camshaft, which is most US cars built before about 1990, you want a synthetic with at least 1,000 ppm of ZDDP. Most mainstream synthetics have less than that.

Why the leak story is wrong

The leak myth comes from early 1970s synthetics that were heavy in esters. Esters swell rubber seals, which is fine in small amounts but at high concentrations reduces the seal's hardness and tensile strength (LN Engineering). Those oils are gone. Seal compatibility testing is now mandatory for API certification, so any oil with the donut on the back has been checked against the seal materials used in engines.

Valvoline says the same thing from the other direction: synthetics don't seek out leaks, and you can switch without a flush. What does happen is that the detergents in a good synthetic clean sludge off a seal that was already failing, and the leak that sludge was plugging shows up. The oil didn't cause it. It revealed it (LN Engineering, Motor Authority).

The counter-argument from Jason Fenske of Engineering Explained, in a video Mobil 1 sponsored, is that a small leak is less serious than the sludge that was hiding it. Sludge restricts oil return, traps heat and speeds up oxidation. A weep from the rear main is a $5 drip pan.

The one caveat: zinc and flat-tappet cams

Older pushrod engines ride the lifter directly on the cam lobe. That's a sliding contact under high load, and ZDDP, the zinc-phosphorus anti-wear additive, is what keeps the lobe from wiping. Roller lifters, which became common through the late 1980s and 1990s, don't have the problem.

Zinc levels in API oils have dropped. Speedway Motors lists about 1,200 ppm for API SH in 1992, about 1,000 ppm for SJ in 1996, and a cap of about 800 ppm from API SM in 2004. The cut came to protect catalytic converters, which phosphorus poisons. Current API SP oils for passenger cars are still at that roughly 800 ppm level.

Whether that matters for a stock flat-tappet engine is argued about. Lake Speed Jr. of Total Seal, who ran FTIR, ICP and shear testing on Mobil 1 Classic Car oil for EngineLabs, concluded modern API SP oils are perfectly acceptable for flat-tappet classics because they add molybdenum (MoDTC) that supplements the lower zinc. Speedway Motors, which sells zinc additives, says 1,000 to 1,200 ppm minimum for flat-tappet engines and 1,400 to 1,500 ppm for the first 500 miles on a new cam.

The safe move is to pick a synthetic that already has 1,000 ppm or more. There's no downside for an engine without a catalytic converter, and plenty of options exist. Mobil publishes zinc and phosphorus for every Mobil 1 grade, which makes it the easiest brand to choose from.

Figures from the Mobil 1 Product Guide dated September 2022. Mobil's own expert page points flat-tappet owners to FS 0W-40 and its 1,000 ppm of ZDDP.

Don't go above about 1,500 ppm without a reason. Speedway Motors notes that ZDDP becomes corrosive and can clog passages past that level, and anything over 800 ppm will shorten a catalytic converter's life. If your older car still has a cat and you drive it daily, the 1,000 ppm oils are the balance point.

What "older" actually means

Age alone doesn't decide anything. A 2010 Camry with 160,000 miles has roller cams, a cat, and a manual that calls for 0W-20. Synthetic is what it was designed for. The question only gets interesting on three kinds of older car.

Flat-tappet engines from before roughly 1990. Zinc, as above.

Carbureted engines that sit. These run rich and collect fuel in the oil. Synthetic handles that better than conventional, but the oil still needs changing once a year regardless of miles, because the fuel and moisture don't boil off on short drives.

Worn engines with low oil pressure. Here the fix is viscosity, not base oil. Motor Authority and LN Engineering both say a thicker grade can help a worn engine hold pressure as a late-stage measure. A high-mileage 10W-40 synthetic gives you the thicker film plus seal conditioners. It won't fix the wear.

Will it cost more

About $17 a change if you do it yourself. AAA's survey put a DIY conventional change at about $28 for five quarts and synthetic at about $45. At a shop, $38 versus $70. Walmart currently lists a 5-quart jug of Mobil 1 Advanced Full Synthetic 5W-30 at $31.97.

Against that, AAA's eight ASTM bench tests found synthetics beat conventional by 47 percent on average across shear, deposits, volatility, cold pumpability and oxidation. On an older engine that's already carrying some sludge and running hotter than new, those are exactly the properties you want.

The higher-zinc grades cost more. Mobil 1 FS 0W-40 and 15W-50 sell in quarts rather than 5-quart jugs at most stores, so budget closer to $10 a quart.

Switching without a flush

Don't flush. LN Engineering warns that solvent flushes dislodge debris that then blocks galleries and the filter, and they stress aged seals. Valvoline says the same: no flush needed unless there's known sludge.

The gentler route is two short intervals. Drain, fill with synthetic, run 1,000 to 2,000 miles, change oil and filter again. The detergents clean gradually and the filter catches what comes loose. After that, go to the normal interval.

Check your manual

The manual gives the viscosity grade and the API spec the engine was built for. On a car from the 1970s or 1980s it will name a spec like API SF or SG that no longer exists; any current API SP oil supersedes it. The grade is often on the oil fill cap too. If the manual says 10W-30 and the engine is tired, a 10W-40 is a reasonable step up.

What to do today

1. Find out if the engine has flat-tappet lifters. Most US pushrod V8s and inline sixes before about 1987 do; a forum for your model will know in one search.
2. Flat-tappet: pick a synthetic with 1,000 ppm or more of zinc. Mobil 1 FS 0W-40, FS 5W-40 or 15W-50 all qualify from Mobil's published guide. Roller cam: any API SP synthetic in the manual's grade.
3. Skip the engine flush. Change oil and filter, run 1,000 to 2,000 miles, then change both again.
4. Check under the car after a week. A new weep means a seal that was already gone, so note it and plan the repair.
5. If the car does under 5,000 miles a year, change the oil every 12 months anyway.

Sources

• Mobil: Mixing Motor Oil to Reach the Right ZDDP Level for Classic Cars
• Mobil 1 Product Guide, September 2022 (PDF)
• LN Engineering: Can You Use Synthetic Oil in Old Cars?
• Speedway Motors: Zinc Additive for Oil, How Much ZDDP Is Enough?
• EngineLabs: Do Classic Cars Actually Need High-Zinc Oil? Lake Speed Finds Out
• Motor Authority: Is Synthetic Motor Oil Bad for Old Cars?
• Valvoline: Can You Mix Synthetic and Conventional Oil?
• AAA Oregon: The Truth About Oil Changes
• Walmart: Mobil 1 Advanced Full Synthetic 5W-30, 5 Quart

Featured image: Past due for an oil change by Chovee, CC BY 2.0. Source