Grease Compatibility Chart: Can You Mix Greases?
Topping up a bearing with whatever grease is on the shelf is one of the most common — and most expensive — maintenance mistakes. Mixing two incompatible greases can break down the thickener structure, soften the grease until it runs out of the bearing, or harden it until it starves the bearing entirely. This guide explains what drives compatibility and gives you a practical chart to check before you mix.
What determines grease compatibility?
A grease is base oil held in a thickener (a soap or non-soap structure) plus an additive package. Compatibility is mostly governed by the thickeners: combining two greases with chemically incompatible thickeners can collapse the structure. Base-oil type (mineral vs synthetic such as PAO or PAG) and additives matter too — PAG-based greases in particular are generally incompatible with mineral and PAO greases.
"Compatible" does not mean the mix performs as well as either original grease — it means the blend will not fail dramatically. Even compatible greases should only be mixed when unavoidable.
Grease compatibility chart
Use this as a quick reference for common thickeners. C = generally compatible, B = borderline (test first), I = incompatible. Always confirm with the manufacturer for your specific products.
| Thickener | Lithium | Li Complex | Calcium | Ca Sulphonate | Polyurea | Aluminium Complex |
|---|---|---|---|---|---|---|
| Lithium | C | C | B | B | I | B |
| Lithium Complex | C | C | B | B | I | B |
| Calcium | B | B | C | C | I | B |
| Calcium Sulphonate | B | B | C | C | B | B |
| Polyurea | I | I | I | B | C | B |
| Aluminium Complex | B | B | B | B | B | C |
What the chart does not tell you
A thickener chart is a starting point, not a guarantee. Other things decide whether a particular mix will hold up in service.
- Additive packages. Two lithium greases can carry very different additives — extreme-pressure (EP) chemistry, anti-wear agents, rust inhibitors, tackifiers or solid lubricants such as molybdenum disulphide. Mixing an EP grease with a non-EP grease dilutes the EP protection, and some additive chemistries react with each other and soften the blend.
- Base oil viscosity. A grease made with a thin base oil for high-speed electric motors and one made with a heavy base oil for slow, shock-loaded pins can share the same thickener. Mixed, the base oil viscosity lands somewhere in between, which may suit neither job.
- Thickeners not on the chart. Clay (bentonite) greases, barium complex and other speciality thickeners behave differently again. Clay greases in particular are commonly rated incompatible with most soap greases. If you cannot identify the thickener, do not mix.
- Food-grade status. An NSF H1 food-grade grease topped up with an ordinary industrial grease is no longer food-grade, whatever the thickeners. In food and pharma plants, keep H1 greases, guns and fittings completely separate.
The same thickener family also varies between suppliers. “Lithium” on two labels tells you about the soap, not the base oil, additive dose or manufacturing process. That is why manufacturers advise checking product against product, not just family against family.
How to check compatibility before you mix
Before you top up or change grease on anything that matters, gather the facts first. It takes a few minutes and can save a bearing.
- Identify both greases. Find the product name of the grease already in the machine — from the maintenance log, the equipment manual or whoever last serviced it. “The yellow one” is not an answer.
- Read both product data sheets. Look for thickener type, base oil type (mineral, PAO, ester, PAG), base oil viscosity at 40 °C, NLGI grade and whether it is an EP grease. If the data sheet does not state the thickener, ask the supplier.
- Check the chart. If the thickeners are marked I, stop and plan a full clean-out. If B, test first. If C, still check base oil and additives before going ahead.
- Do a simple bench check. Mix equal parts of the two greases in a clean container, with a sample of each on its own alongside. Keep all three warm — near the equipment’s running temperature — for a day or two. If the mix is visibly softer, runnier or separates more oil than either original, treat the pair as incompatible.
- For critical equipment, ask for a lab check. Grease compatibility is formally assessed under ASTM D6185, which compares the mixture’s consistency, dropping point and shear stability with the original greases. Your supplier can advise whether it is worthwhile.
How to switch greases safely
- Purge, don't top up. Pump fresh grease through the bearing until only the new grease emerges from the relief or seal.
- Increase frequency temporarily. Re-grease at shorter intervals for the first few cycles to flush residual old grease.
- Clean on a full service. For critical equipment, dismantle and wash out the old grease with a suitable solvent before repacking.
- Label the change. Record the new grease on the equipment so the next technician doesn't reintroduce the old type.
Purging without damaging seals
Purging is the right way to change grease, but done carelessly it causes damage of its own. A hand grease gun can build far more pressure at the nipple than a lip seal can hold back. Pumping hard into a housing with no escape route can push seals out of their seats or collapse the shields on a bearing.
- Open the relief first. Remove the drain or relief plug on bearing housings before purging so old grease has somewhere to go. Run the machine briefly afterwards to expel the excess, then refit the plug.
- Pump slowly. Steady strokes give the old grease time to move. If the gun suddenly becomes hard to pump, stop and find out why rather than forcing it.
- Watch the colour. Many greases are dyed. Keep going until the grease emerging from the relief or seal lip looks like the fresh product.
- Do not overfill. A housing packed solid churns the grease, heats up and pushes grease past the seals. A temperature that rises for a few hours after purging and then settles is normal; one that keeps climbing means too much grease.
- Sealed-for-life bearings cannot be purged. If a bearing has no grease fitting, it was designed to run on its factory fill. Do not drill it or force grease past the shields.
Signs of an incompatible mix
Watch for grease that has softened and is leaking from seals, grease that has hardened or crusted at the fitting, rising bearing temperatures, or unusual noise shortly after a grease change. Any of these after switching products points to incompatibility — stop, purge, and standardise on one grease.
Keeping greases separate: guns, cartridges and storage
Most incompatible mixes are not planned. They come from one workshop gun used for every grease, an electric motor factory-filled with polyurea and topped up with lithium, a change of maintenance contractor, or a bucket refilled from a different product. The cheapest fix is at the store and the grease gun.
| Practice | Why it matters |
|---|---|
| One gun per grease, labelled or colour-coded | Stops the last product’s residue being pushed into the next bearing |
| Use cartridges where possible | Sealed cartridges keep out dust and water and remove the guesswork of filling from open buckets |
| Wipe the nipple and gun coupler before every shot | Dust on the fitting goes straight into the bearing with the grease |
| Keep lids on buckets and kegs; use a clean scoop or pump | Open containers in a busy workshop collect dust, water and the wrong grease |
| Tag each machine with its grease | The next technician sees at a glance what goes in |
| Store under cover, away from heat and direct sun | Heat speeds up oil separation and ageing; rainwater in an open container spoils the grease |
| First in, first out | Use older stock first and note the manufacturing date when stock arrives |
A thin film of free oil on the surface of grease in a container is normal — grease releases a little oil as it stands, which is also how it lubricates inside a bearing. Smooth the surface flat after each use rather than digging a hole in the middle, which encourages oil to pool. If a container shows a deep pool of oil, a change in colour or a hard skin, check with the supplier before using it.
Standardise on fewer greases
Most compatibility problems disappear when a site or fleet cuts down the number of greases it stocks. A typical plant or transport fleet can cover most of its points with a handful of products:
| Duty | Grease type | Haruto range |
|---|---|---|
| General bearings and multipurpose points | Lithium | Lithium MP (NLGI 2 / 3) |
| Hot or heavily loaded bearings, wheel hubs | Lithium complex | Lithium Complex (NLGI 1 / 2 / 3) |
| High-load bearings and gear couplings | EP grease | EP Series (NLGI 00 to 3) |
| Wet areas, marine and steel plants | Calcium sulphonate | Calcium Sulphonate (NLGI 1 / 2 / 3) |
| Electric-motor and sealed-for-life bearings | Polyurea | Polyurea (NLGI 2 / 3) |
| Chassis, pins and suspension | Water-resistant chassis grease | Red Gel HAX (NLGI 3) |
| Food and pharma machinery | Food-grade grease | Food-Grade (NLGI 1 / 2) |
Lithium and lithium complex sit in the same compatible family, so many sites move everything they can onto lithium complex and keep polyurea and food-grade greases strictly apart. See the full Haruto grease range and the NLGI grade guide when choosing consistency. Where a machine’s manual names a specific grease, the manual decides.