Switching to synthetic oil in a high-mileage vehicle

Drain the old oil, pour in the synthetic, drive. That's the whole procedure. There's no flush, no transition blend and no mileage past which it stops being a good idea. Porsche has factory-filled with synthetic since 1996, and a 150,000-mile engine isn't any more fragile than the day it left the lot.

The only decisions are which synthetic, and whether to spend the extra on a high-mileage version. Here's what the makers publish and what it does for you.

Will it cause leaks

No, but it may reveal one. Seal compatibility testing is mandatory for API certification, so any oil with the API donut has been tested against the elastomers used in engine seals (LN Engineering). Valvoline says it plainly: synthetics don't seek out leaks, and you can switch without a flush.

The leak reputation comes from two things. The first is 1970s ester-heavy synthetics that swelled seals too much; those formulations are gone. The second is real and still happens: the stronger detergents in a synthetic clean sludge off a seal that was already cracked, and the drip that sludge was plugging shows up a week later (LN Engineering, Motor Authority).

That's a seal that needed replacing anyway. LN Engineering's line is that sludge is not a sealant. It restricts oil return, traps heat and speeds up oxidation. A rear main seal weeping onto the driveway is cheaper than a sludged-up oil pickup.

What a high-mileage synthetic actually adds

Three things, by the makers' own descriptions: seal conditioners, more detergent, and often a slightly thicker base oil. Mobil's page for Mobil 1 High Mileage says its additives help combat leaks, sludge and general wear, and that a higher base oil viscosity helps reduce leaks. Motor Authority describes the typical formula as a swelling agent that conditions seals plus a slightly heavier cut that can reduce consumption.

Here's the honest limit. Mobil 1 High Mileage 5W-30 and standard Mobil 1 5W-30 have almost identical published viscosities, so the "thicker" effect within a grade is small.

Figures from Mobil's product data pages. The real viscosity step is going from 5W-30 to 10W-30, not from standard to high-mileage within a grade.

What you can't see in the table is the seal conditioner, and no maker publishes how much is in there or what it is. The high-mileage version carries the same API and ILSAC approvals as the standard oil, so you give up nothing by choosing it. You just can't verify the extra.

The 75,000-mile line is marketing, not chemistry. Mobil, Valvoline and the rest all use it. An engine at 60,000 miles with a weep doesn't care about the number.

Whether to flush first

Don't. LN Engineering, which builds engines, is specific about why: solvent flushes dislodge debris that then blocks oil galleries and the filter, they reduce film strength while they're in there, and they stress old seals. Valvoline says a flush isn't needed as a rule, only where there's known sludge.

The gentler approach works better. Change the oil and filter, run 1,000 to 2,000 miles, then change both again. The detergents in the synthetic clean gradually and the filter catches what comes loose. After the second change, go to your normal interval.

If you pull the fill cap and see a baked brown crust on the inside, that's sludge. Do three short intervals instead of two, and cut the filter open after the first one if you want to see what came out.

When to be careful with a thicker grade

A worn engine with low hot oil pressure can benefit from stepping up a grade, say 5W-30 to 10W-30 or 10W-40. Motor Authority and LN Engineering both describe this as a late-stage measure that helps maintain pressure, not a repair.

Don't do it on a modern engine with variable valve timing or cam phasers unless the manual lists the thicker grade as an option. Those systems are tuned for the oil's flow rate. On a 1990s or early-2000s pushrod engine with a manual that lists 10W-30 as acceptable, it's a reasonable move.

What it costs

About the same as any synthetic. Walmart lists a 5-quart jug of Mobil 1 High Mileage 10W-30 at $31.97, the same price as standard Mobil 1 Advanced Full Synthetic 5W-30. The high-mileage premium at retail is usually zero to a couple of dollars.

AAA's cost survey puts a DIY synthetic change at about $45 for five quarts against $28 for conventional, and a shop change at $70 against $38. That's about $64 a year extra at normal intervals, by AAA's math.

What you get for it, from AAA's eight ASTM bench tests, is 47 percent better average performance across shear stability, deposits, volatility, cold pumpability and oxidation resistance. On a high-mileage engine the deposit and oxidation numbers are the ones that matter, because that engine runs hotter and dirtier than it did new.

The drain interval is where the money comes back. Mobil 1 High Mileage is guaranteed for 15,000 miles or a year, and the Extended Performance High Mileage version for 25,000 miles or a year. Both pages tell you to follow the oil life monitor or manual if the car is under warranty. On a 150,000-mile car that's long out of warranty, a 10,000-mile interval on synthetic costs less per mile than 5,000-mile changes on conventional.

Check your manual

The manual gives the grade and the spec, and the grade is usually on the oil fill cap. If the manual lists more than one grade with a temperature chart, the thicker one is your option for a worn engine. Match the spec first: a 2015 or newer engine wants API SP or later for the LSPI protection, and every current Mobil 1 High Mileage grade carries SP and SQ.

What to do today

1. Check the oil level and look under the car. Note any existing drips so you know what's new after the switch.
2. Buy a high-mileage synthetic in the manual's grade. Mobil 1 High Mileage carries API SP and SQ with ILSAC GF-7A; Valvoline MaxLife and Pennzoil Platinum High Mileage are the equivalents from the other majors, so check the back label for the same spec.
3. Change oil and filter. No flush, no additive.
4. Drive 1,000 to 2,000 miles and change oil and filter again. Then go to the normal interval, or 10,000 miles if the car is out of warranty and does mostly highway miles.
5. Recheck the driveway after a month. A new weep is a seal that was already gone. Plan the repair, and keep running synthetic.

Sources

• Mobil 1 High Mileage 5W-30
• Mobil 1 5W-30
• Mobil 1 Extended Performance High Mileage product data sheet
• LN Engineering: Can You Use Synthetic Oil in Old Cars?
• 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 Super High Mileage listing with Mobil 1 High Mileage 10W-30 5 Quart price
• Walmart: Mobil 1 Advanced Full Synthetic 5W-30, 5 Quart

Featured image: 2020/366/150 The Big 200000 by cogdogblog (CC0). Source