Synthetic Grease Lubricants
A lubricating grease is a base oil held in a thickener, plus additives. In a synthetic grease the base oil is made in a plant rather than distilled from crude oil. Common synthetic oils are polyalphaolefin (PAO), esters, and polyalkylene glycol (PAG). The thickener is a separate choice. It can still be a lithium soap, a lithium complex, polyurea, or a clay. Calling a grease synthetic describes the oil, not the soap and not the color.
Synthetic oil is used when a mineral oil would oxidize, evaporate, or thicken too soon. A hot bearing, a long relube interval, a cold start, or a wide swing between those temperatures is the usual reason. The oil stays fluid at the low end and resists breaking down at the high end. That does not make every synthetic grease a high-temperature grease. Temperature rating comes from the whole formula, especially the thickener. A lithium complex or polyurea thickener is what lets many industrial greases stay in place after a simple lithium soap would melt.
The NLGI grade is still the stiffness. NLGI 2 is the common multipurpose body. A softer grade, such as NLGI 1 or 0, pumps more easily through long lines. A harder grade stays put in a slow, heavily loaded joint. Additives are specified on their own line: extreme pressure for sliding contact, antiwear for rolling contact, rust inhibitors, and tackifiers that help the grease cling. None of those words is implied by the word synthetic.
Synthetic grease is not silicone grease. Silicone grease is a different oil, used on rubber and plastic so a petroleum oil does not swell the seal. It is a poor lubricant for most metal bearings. Synthetic grease is also not the food product called choice white grease. Choice white grease is an animal fat sold as a commodity. White lithium grease is a lubricant. The names share the word grease and nothing else.
Do not mix thickeners in one bearing. Lithium, lithium complex, polyurea, calcium, and clay do not all agree with each other. A new grease that looks the same color can still soften or harden the old charge. If the thickener is unknown, clean the bearing before a change. GREASE.ORG explains these distinctions so a buyer can read a data sheet. This page is not a catalog and it does not quote a brand.
Read three lines on any tube or pail before you buy: base oil type, thickener, and NLGI grade. Then match the dropping point, the oil viscosity, and the water resistance to the machine. A synthetic oil is worth the cost when the bearing runs hot, the interval is long, or the OEM names a synthetic spec. It is wasted money on a hinge that a plain lithium NLGI 2 already serves.
