Gear Oil

Since the development of the hypoid rear axle (gears working at a right angle) by Packard in the ’20s, there has been major concern over gear oil and the proper formulation. Due to the high degree of sliding between the ring and pinion and the incorporation of spider gears for easy turning, a strong lubricant with good properties is required. These are usually referred to as an EP lubricant for extreme pressure.

Gear oil formulation is a difficult balancing act between the various requirements of the application. Precise formulations are proprietary, and the manufacturing process for individual additives and the purity are highly guarded and make the product brand specific.

A fluid test with OSA may be the best $14.95 you will ever spend on your Pontiac

Gear oil needs to contain anti-oxidants and anti-corrosion additives, cleanliness dispersant, and seal compatibility and anti-foam compounds. If a limited-slip differential is employed, additional friction-control additives are required to prevent chatter and noise.

Gear oil is identified by the American Petroleum Institute (API) designation that begins with the letters GL followed by a hyphen and a number. Though GL-1 to GL-4 are now obsolete, most applications call for GL-5 or better test criteria. In addition there is a viscous weight assigned to the oil, such as 75W-90. Early Pontiac differentials used a 90-weight oil, which was thicker than the commonly produced today multi-viscosity 75W-90 for light-duty vehicles.

When it comes to lubricants, the W stands for Winter (the same holds true for motor oils). Thus a 75W-90 or 80W-90 GL-5 gear lubricant will not thicken as much in the winter (when compared to straight 90-weight) and will offer the viscosity-breakdown protection of a 90-weight when at operating temperature. This is the ideal differential lubricant for our readers with older Pontiacs. The multi-viscosity lubricant will allow for less wear during cold operation than what was employed years back.

Some better brand gear oils already have a limited-slip additive mixed in and will require nothing else. The bottle will identify if it does. These fluids can safely be employed in an “open” rearend or one with no traction-sensing clutches.

Myths and Facts

Many manufacturers may claim a “fill-for-life” status on their transmission fluid. That would only be under ideal operating conditions, which few vehicles experience. Short trip cycles with many shifts, high engine-operating temperatures, and wear in the gearbox will all degrade the fluid and are the reasons it should be changed, or at the least analyzed.

In the ’70s, GM claimed the very popular sniff test (removing the dipstick and smelling the transmission fluid) was unreliable. The corporation stated that Dextron and later fluids would take on a brown appearance instead of red and a burnt smell, but will still be functional. GM claimed that if the transmission shifted fine, no service would be required. It seems this statement was more a response to satisfy customer complaints than sound engineering. The only way to actually determine if a fluid is still usable is through chemical analysis.

Since almost every Pontiac automatic transmission has a cooler that is part of the vehicle’s radiator, any problem with the engine’s cooling system will impact the transmission fluid life and operation. An engine that is overheated just once will ruin the transmission fluid. For this reason if your Pontiac ever has a hot-running problem, it is important to service the automatic gearbox after the problem is corrected.

Fluid operating temperature versus life is not linear. Once the fluid reaches near 300 degrees F, its useful life is measured in tens of miles—not tens of thousands. A stuck thermostat in the engine and a mechanic not recognizing its impact on the fluid ruins more automatic transmissions than anything else.

Many old-time mechanics say that if a car has never had the transmission fluid changed, you shouldn’t renew it because the unit will fail afterwards. If the transmission has been neglected and is full of contaminants, the new fluid with its fresh detergents can dislodge some foreign material and cause a potential problem. The thing to remember is that if you don’t change the fluid, the transmission will eventually fail anyway. It’s a rare instance that it is hastened with the new fluid. The important point is you shouldn’t allow the transmission to get to this state.

More Considerations

Most, if not all, Pontiacs don’t have a drain plug on the torque converter, so even when dropping the pan to change the fluid, you will not be getting it all out. It will take two or more changes depending on the size of the torque converter to have all fresh fluid in the gearbox. For this reason, you may want to consider an aftermarket transmission pan with a drain plug, or simply drill a hole and weld a drain plug into your OE pan. This way the fluid can be exchanged easily for all new.

Regardless of the fluid being considered, a yearly analysis will prove to be a valuable tool in keeping your Pontiac on the road. As an aside, if the day comes when you need to part company with your Tin Indian, a file of fluid reports over the years could help you command a higher price and remove a good deal of uncertainty from the buyer about the Pontiac’s condition. A fluid test with OSA may be the best $14.95 you will ever spend on your Pontiac.

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