Replacement of Gear Lubes Can Prevent Expensive Driveline Problems

The lubricants that are used in manual transmissions, transaxles, differentials and transfer cases include a variety of different oils. Many transmissions, transaxles and transfer cases use gear oil with viscosities ranging from 75W-90 up to 85W-140. Others use ATF (Dexron II/III, Mercon V, etc.) or even motor oil (10W-30 or 10W-40). Most differentials, by comparison, use hypoid gear oils that also contain extra amounts of "extreme pressure" (EP) additives. Limited slip differentials also require their own special additives.

Most of these lubricants are long-lived and hold up well for tens of thousands of miles. But none will last forever. The combination of heat, shearing action and oxidation eventually breaks down the oil and reduces its ability to lubricate and protect.

Normal wear inside the gearbox and differential also produces metallic debris that ends up in the oil. Since there’s no filter to remove these contaminants, the fluid becomes more and more abrasive as the miles add up. The only way to get rid of the contaminants and restore the lubricating qualities of the oil is to drain and replace the fluid. The question is when.

Many vehicle manufacturers do not publish a recommended service interval for manual transmissions, transaxles, transfer cases or differentials, but some do. Chrysler, for example, says the transmission and transfer case on 1999 Jeep Cherokees should be drained and refilled every 30,000 miles or 24 months. Other manufacturers, though, say only to inspect the fluid level periodically and add as needed to maintain the proper level.

The oil level inside a transmission or differential is critical for proper lubrication because there's no oil pump to route the oil where it's needed. The oil is churned by the whirling gears, which "splash lubricates" the moving parts.

If the fluid level gets too low because of a leak, therefore, the bearings and gears won't get enough lubrication. The result can be galling, seizure and total destruction of the unit. Oil is also necessary to cool gears and bearings. The total oil capacity of most manual transmissions, differentials and transfer cases isn't very much (typically a couple of quarts or less), so it doesn't take much fluid loss before parts start running dangerously hot.

If a transmission or differential is whining and making noise, it's too late to add oil. The damage has already been done. Adding a higher viscosity oil may quiet it for awhile, but once wear has taken its toll on the gears and bearings, there's no magic cure other than to overhaul the unit and replace the worn parts.

Most conventional oils thicken as the temperature drops. This increases friction, drag, fuel consumption and the effort needed to shift gears. During subzero weather, a heavy gear oil inside a manual transmission can make the shifter feel stiff and clunky. The gears may even grind when changing gears until the vehicle warms up. The cure here is to replace the conventional gear oil with a synthetic gear oil.

Synthetic gear oils have a lot of advantages compared to conventional petroleum-based oils. Synthetics are more stable, flow more easily at low temperatures, reduce friction and operating temperatures, improve fuel economy and generally provide superior all-round lubrication and protection under a wide range of operating conditions. But they are also more expensive.