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We don’t typically see too much grease on modern vehicles, but only because it’s hidden inside maintenance-free replace-it-when-it-breaks components. On older vehicles, aftermarket parts, farm equipment and other gear, you’ll find grease fittings everywhere. There are many types of grease, but can you mix grease?

For example, you might have heard molybdenum or marine grease is better than multi-purpose grease, so why not just use that the next time you get a new grease cartridge? In a pinch, if you’ve got wheel bearing grease in a tub, can you just put some of that in your ball joints? In either of these situations, you might mix grease in a way that leads to worse performance. In lube-speak, poor performance translates directly to accelerated wear and premature breakdown.

Why You Can’t Mix Grease — Usually

At its most basic, grease is a homogeneous blend of a synthetic or mineral oil and a thickener. Other additives include anti-wear agents, antioxidants, tackifiers and friction modifiers. We already know synthetic oils and mineral oils are perfectly compatible, which is why there’s no problem using an appropriate grade synthetic oil in any engine you can imagine. Still, that doesn’t mean we can mix grease, whether it’s synthetic, mineral or even bio-based.

The problem is with the thickening agents used, such as aluminum, calcium, lithium and polyurea. Testing by major grease and bearing manufacturers reveals some grease types are incompatible. Lithium 12-hydroxy is incompatible with aluminum complex and calcium complex thickeners, but moderately compatible with calcium sulfonate, a typical marine grease thickener. At the same time, lithium complex and lithium 12-hydroxy thickeners are compatible with each other.

  • General purpose lithium grease is what we usually think of when considering a new grease cartridge, and that’s the stuff that you’ll usually find in chassis components, like ball joints and tie rod ends. Lithium-based grease is also the most common.

  • On the other hand, marine grease is used where water-washout is a deeper concern, such as boat trailer wheel bearings, inboard prop-shafts and powersports machines. Marine grease’s calcium-based thickening agents are water-resistant and tackier, which helps it stay in place even when submerged.

Mixing grease, though, could be disastrous. As the various thickening agents interact, they might lead to grease hardening or oil dropout. In either case, your bearings, shafts, joints or other moving parts could be left unprotected, open to corrosion, prone to wear or apt to complete failure.

Don’t Mix Grease, but Change If Needed

The general rule is not to mix grease, but that doesn’t mean you can’t switch to another grease if your use demands it. Most automotive components are lubricated with lithium grease, but if you use your 4X4 or AWD mainly for off-roading, switching to a calcium marine grease might be beneficial.

The key is to make the switch as close to 100 percent as possible. If you can, start with completely clean parts and new grease. Otherwise, greasing more often will flush out the old incompatible grease faster. Still, some joints will not flush, which could be risky.

Check out all the chemical products available on NAPA Online or trust one of our 16,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on types of grease, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

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