AMSOIL SAE 50 Long-Life 100% Synthetic Transmission Oil

Product overview

Heavy-duty manual transmissions in line-haul service run a long time between fluid changes, so the oil has to hold its viscosity for the duration. AMSOIL SAE 50 Long-Life Synthetic Transmission Oil is built for equipment that specifies an SAE 50 transmission oil and runs extended intervals.

AMSOIL put it in line-haul trucks and disassembled a transmission after more than 500,000 miles. A calibrated ASTM rater inspected the critical components and found outstanding wear protection.

What the formula does

• Resists viscosity loss from mechanical shear, so the lubricating film stays intact under high load and high torque.
• A high viscosity index means easier shifts and less fluid drag when it is cold, plus thermal stability that keeps the transmission clean when it runs hot.
• Foam inhibitors keep churning gears from aerating the oil, which is what leads to metal-to-metal contact and wear.

Where this fluid fits

Use in manual transmissions and gearboxes that specify an SAE 50 lubricant, including:

• Heavy-duty highway truck transmissions
• Line-haul and vocational fleet equipment

What a 500,000-mile drain interval demands of an oil

AMSOIL SAE 50 Long-Life Synthetic Transmission Oil is formulated for manual transmissions in high-torque, heavy-duty line-haul trucks and other commercial applications, following the original equipment manufacturer's synthetic drain interval up to 500,000 miles or five years, whichever comes first. An interval that long puts the weight on viscosity stability, because an oil that thins out early stops carrying the film the gear teeth depend on.

Resistance to viscosity loss from mechanical action is the center of the formula. In the 60-hour KRL test (CEC L-45-A-99) it recorded a 4.97 percent change in kinematic viscosity at 100°C. Fresh oil measures 17.5 cSt at 100°C and 135.2 cSt at 40°C.

The line-haul teardown and what the rater found

The oil was installed in line-haul trucks and placed into real-world service, and one of those transmissions was torn down past the 500,000-mile mark for inspection by a calibrated ASTM rater. The gears and the synchro friction material showed minimal wear at that mileage, and the output seal had not leaked and still held 100 percent of its grease. Bench results line up with the teardown, at a 0.38 mm scar in the four-ball wear test (ASTM D4172 at 40 kg, 75°C, 1200 rpm, one hour).

Cold-morning shifts, fluid drag, and foam

A viscosity index of 142 delivers better high-temperature and low-temperature performance than conventional fluids, and in cold weather that shows up as easier shifts and reduced fluid drag, which helps fuel economy. Brookfield viscosity at -40°C measures 117,483 cP, and the pour point is -42°C (-44°F). At the hot end, thermal stability holds off chemical breakdown and sludge formation so the transmission runs cleaner.

Foam is the quieter problem inside a gearbox. Churning gears pull air into the oil, and when those bubbles collapse between gear surfaces the result is metal-to-metal contact and added wear. Foam inhibitors keep a strong lubricating film in place, and the oil returns 0/0, 0/0, 0/0 in the ASTM D892 foam tendency test.

Heavy-duty coverage, not a passenger-car gearbox oil

The application is manual transmissions requiring an SAE 50 lubricant in heavy-duty equipment, anchored by fleet specifications such as Eaton PS-164 Rev. 7, Mack TO-A Plus, and ZF Freedomline. A car or light-truck manual box that calls for a GL-4 lube takes 75W-90 Manual Transmission and Transaxle Gear Lube, and a powershift transmission running wet brakes and hydraulics takes Powershift SAE 50. Sharing an SAE 50 label is not the same as sharing an application.

Specifications SAE 50 Long-Life Transmission Oil covers

This oil covers applications specifying any of the following:

• Eaton PS-164 Rev. 7
• API GL-1 and MT-1
• Navistar/International TMS 6816
• Mack TO-A Plus
• ZF Freedomline
• Meritor 0-81
• Rockwell, Eaton, and Fuller CD-50 (PS-081)

Coverage as published in the AMSOIL data bulletin, checked July 2026. AMSOIL revises coverage when a formula changes, so confirm against AMSOIL's current data bulletin before ordering.

Service life

Follow the original equipment manufacturer's drain interval for synthetic oil, up to 500,000 miles or five years, whichever comes first.

Good to know

• The formula is compatible with seals, which helps prevent leaks and extends seal life.

Typical technical properties

Typical properties published by AMSOIL, checked July 2026. These are representative values rather than a specification, and they change when AMSOIL updates a formula.

Package sizes and product codes

Select a product code to open that package size on the AMSOIL product page.

Product documents

AMSOIL publishes the full technical detail for this product in these documents.

• SAE 50 Long-Life Transmission Oil product data bulletin (PDF)
• SAE 50 Long-Life Transmission Oil safety data sheet (PDF)

Where to order

Confirm this formula is the match for the vehicle or equipment in the AMSOIL product guides, then order SAE 50 Long-Life Transmission Oil online. Existing members can sign in to apply the discount automatically.

Details and questions