Synthetic vs Conventional Gear Oil: Which One Should You Buy?

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Below: what the synthetic listings state about viscosity, thermal stability and limited-slip use, four synthetic examples from the listings, and what to check before a fill.

Synthetic vs conventional gear oil: what the listings state

Every gear oil example in this guide is described as synthetic or full synthetic in its listing. The Valvoline SynPower listing states that its fluid helps provide thermal stability for cleanliness and longer service life, and that it contains additives to assist in protecting gear teeth from rust and corrosion and to reduce chattering in limited-slip differentials. The Valvoline Flexfill listing states a full synthetic formula for high and low temperature protection, with an extreme pressure additive for load carrying capacity and wear protection, and thermal stability designed to protect oil from high temperature breakdown.

Viscosity is the first figure to match to the manual. The listings here state three grades: 75W-90, 75W-140 and 0W. The Valvoline SynPower and Royal Purple listings both state 75W-90, the Valvoline Flexfill listing states 75W-140, and the Lucas Oil listing states a viscosity grade of 0W for its stabilizer. A grade from one listing never describes another listing, and the manual for the vehicle decides which grade belongs in a given differential.

The GL rating is the second figure to match. The Valvoline SynPower listing recommends its fluid for conventional and limited slip differentials and non-synchronized manual transmissions where API GL-5 or API GL-4 fluid is specified. No other listing in this guide states a GL rating, so a GL figure from that listing does not apply to the other three products. If the manual names a GL rating, that rating, not the brand on the bottle, decides the choice.

Flash point is stated for two of the four listings, and the two figures differ. The Valvoline SynPower listing states a flash point of 302 °F, while the Valvoline Flexfill listing states a flash point of 288.5 °F. The Lucas Oil and Royal Purple listings state no flash point, so no comparison on that figure is possible for those two. A flash point alone does not make one fluid better for a given differential; it is one stated figure among several.

Gear oil examples from the listings

The Valvoline SynPower 75W-90 full synthetic gear oil is the most directly described gear oil in this guide. Its listing states thermal stability, additives for gear teeth protection against rust and corrosion, reduced chattering in limited-slip differentials, and a recommendation for conventional and limited slip differentials and non-synchronized manual transmissions where API GL-5 or API GL-4 fluid is specified. It is sold as 128 fluid ounces, with a flash point of 302 °F stated.

The Valvoline Flexfill 75W-140 full synthetic gear oil suits a different stated use. Its listing states a full synthetic formula for high and low temperature protection, an extreme pressure additive for load carrying capacity and wear protection, and thermal stability designed to protect oil from high temperature breakdown. It is sold as a case of 4 quarts, with a flash point of 288.5 °F stated, and its listing states that it works in tight spaces and makes gear oil easier to use.

The Royal Purple Max Gear 75W-90 synthetic gear oil is the listing with the least detail in this guide. Its bullets state only that the best performance follows the manufacturer recommendations in the vehicle owner manual. Its spec table states a viscosity of 75W-90, a liquid volume of 1 quart, an item weight of 16 ounces, a synthetic material, and a specific use of improving power and protection for heavily loaded gears in trucks, automobiles and marine engines. No flash point is stated for this product.

The Lucas Oil 10131 Pure Synthetic Oil Stabilizer is not a standalone gear oil in the same sense as the other three. Its listing describes a synthetic oil additive that stabilizes oil pressure in modern electronically controlled engines, protects heavy duty, high-performance transmissions, gear boxes and engines, quiets noisy transmissions, and reduces heat and wear. It is sold as a 1 gallon bottle, weighs 7.4 pounds, and states a viscosity grade of 0W. Read it as an additive for a system that already has an oil, not as a direct replacement for a gear oil grade the manual names.

What to check before a fill

• Check the viscosity grade the vehicle manual names, and match it to the grade a listing states: 75W-90, 75W-140 or 0W in this guide.
• Check the GL rating the manual names, and note that only the Valvoline SynPower listing here states API GL-5 or API GL-4 use.
• Check the fill size a listing states, from 1 quart to 1 gallon and a case of 4 quarts, against the amount the job needs.
• Check the use a listing states, from limited-slip differentials to heavily loaded gears and tight spaces, before choosing a product.
• Follow the vehicle owner manual for the service, and do not substitute a stabilizer for a gear oil grade the manual names.

FAQ: garage tools

Is synthetic gear oil better than conventional gear oil?

Not in every case. The synthetic listings here state thermal stability and gear protection, but the right choice is the fluid the vehicle manual specifies for that differential. Where a manual specifies a conventional oil, that specification decides the fill.

Can synthetic and conventional gear oil be mixed?

The listings reviewed here do not state mixing, so no mixing claim is made in this guide. Follow the vehicle owner manual for the fluid to use.

What viscosity grades do these listings state?

The listings state 75W-90 for the Valvoline SynPower and Royal Purple products, 75W-140 for the Valvoline Flexfill product, and 0W for the Lucas Oil stabilizer. Match the grade to the manual, not to another listing.

What do GL-4 and GL-5 mean in this guide?

They are the API gear oil ratings named in the Valvoline SynPower listing, which recommends its fluid where API GL-5 or API GL-4 fluid is specified. For the full comparison, see our GL-4 vs GL-5 gear oil guide.

Is an oil stabilizer the same as gear oil?

No, on the evidence of the Lucas Oil listing. That listing describes a synthetic oil additive for transmissions, gear boxes and engines, sold as a 1 gallon bottle, rather than a standalone gear oil grade for a differential.

How much gear oil do I need for a fill?

The listings state sizes from 1 quart to 1 gallon and a case of 4 quarts, but no listing states a vehicle fill capacity. Check the vehicle owner manual for the amount, and use a fluid transfer pump for gear oil for a clean fill.

Related: our best gear oil GL-5 picks and the engine and shop equipment hub.