The 10 Best Oil Additives

For an older engine that smokes a little, leaks a little or rattles on a cold start, Lucas Heavy Duty is the cheap, familiar fix. It's a thick, all-petroleum liquid that helps oil cling to moving parts, and it can slow oil burning in a tired motor. Lucas says it's fine in new engines and in gearboxes too. That thickness is also the downside: a tight modern engine built for thin oil is better off without it.

Made for the thin oils most newer cars call for, Lucas Pure Synthetic is a lighter take on the classic Lucas stabilizer. The company pitches it for computer-controlled engines, where it aims to steady oil pressure and hush the lifter tick some engines make at startup. It can also go into manual gearboxes and differentials. All of that rests on the maker's word, since no outside lab has published results for it.

Popular with owners of German cars, Liqui Moly Cera Tec carries tiny ceramic particles that settle into rough metal surfaces to cut friction. Liqui Moly, which makes it in Germany, says it mixes with any motor oil, leaves no deposits, passes through filters and is safe with diesel soot filters. One bottle goes in at an oil change. The friction claims come from the maker alone, and the oil can look darker afterward.

Ford Power Stroke owners with rough cold starts are the main audience for Hot Shot's Secret Stiction Eliminator. Stiction is gummy residue from burnt oil that makes the oil-driven fuel injectors in these diesels stick. The maker calls for a heavy first dose to clean, then a smaller dose at later changes, and says gas engines can use it too. Its wear-reduction figure comes from a lab test the maker doesn't name, so treat it as a claim.

Marvel Mystery Oil has been sold for about a century, and it's the red bottle many home mechanics keep on the shelf. It goes in the oil to quiet valve clatter and in the gas tank to clean carburetors and injectors. Marvel scales the dose by model year, with older engines getting a full bottle and newer ones much less. It comes as a six-pack here, and its label carries a California warning about one ingredient.

Classic and muscle cars with flat-tappet camshafts need more zinc than modern oils carry, and Rislone Hy-per Lube ZDDP puts it back. Its dose runs from a full bottle for gas engines from 1996 and older to a quarter bottle for 2005 and newer. Rislone makes it in the U.S. and doesn't intend it for break-in. The trade-off is phosphorus, which can shorten a catalytic converter's life, so ask Rislone first for a daily driver.

Anyone who just rebuilt an engine or put in a new camshaft should look at Red Line Break-In. It adds the zinc and phosphorus a fresh cam needs in its first miles, plus cleaners that help piston rings seat. One bottle treats up to 12 quarts, so it can cover a second engine or a second oil change. It's made for conventional oil, and Red Line says oil treated with it should be changed sooner than usual.

BG MOA usually reaches drivers through repair shops, which add a can at the oil change, but it's sold on its own as well. BG says it fights sludge, varnish and sticking piston rings, and helps an engine burn less oil. The can states that it's safe with current oil standards and with small turbo engines prone to early ignition knock. BG publishes little beyond those claims, so there's not much outside detail to judge it by.

Archoil AR9100 is a friction modifier with a big following among Ford 6.0 and 7.3 Power Stroke owners fighting stiction. One bottle treats one of those engines, and Archoil says it also suits gas engines, motorcycles, differentials and power steering. It tends to quiet an engine and ease winter starts soon after it goes in. It costs a lot for one bottle, though, and that's hard to justify in a healthy engine.

For a used car with an unknown past, Liqui Moly Pro-Line Engine Flush gives the inside of the engine a one-time cleaning before fresh oil goes in. Pour it into the warm old oil, let the engine idle for about ten minutes, then drain everything and change the filter. Liqui Moly says it's gentle on seals and safe for catalytic converters. It does nothing between oil changes, and the engine must never be driven with it in the oil.