In many cases, especially on all-wheel-drive vehicles, the correct fluid is determined not just by weight but according to manufacturer-specific specifications, such as on some Honda and BMW vehicles.
It’s always important to consult the service manual and check the manufacturer’s specifications. It is also a good idea to verify that a service facility has done so and is using the correct fluid.
The costs are still generally within the same price range. However, sometimes extra time is needed to round up the necessary fluids if they aren’t on hand.
The Costs of Differential Fluid Service
The cost of changing differential fluid ranges from around £60 to £145. The price is usually figured at half an hour of labour and then the cost of fluids. Most shops, including dealerships, use a set price rather than the book-time, plus the parts that the estimates for other services are calculated with.
In some cases, there is one price for all-wheel-drive vehicles and another price for four-wheel-drive trucks, which tend to have a larger gear oil capacity. In many cases as well, the operation is different between the two.
Most all-wheel-drive vehicles have a simple drain plug for the differentials. On the other hand, many four-wheel-drive axles require the more laborious removal of the drive gear cover to drain the fluid.
Sometimes, there is also an additional charge on top of the differential service cost to service a limited-slip differential if special friction-modifier additives need to be used. It’s important to use the correct additive if necessary, or the clutches in a limited slip differential can bind up or operate intermittently rather than smoothly.
An example would be a 2004 Dodge 2500 four-wheel-drive truck with a limited-slip rear end, which needs a 4-oz of limited-slip additive costing about £16, in addition to normal differential fluid.
Differential Fluid Service
The differential in a vehicle takes the input of engine power from a driveshaft and splits it into two driving wheels on the axle using heavy steel ring and pinion gears. These are connected to a steel frame housing the spider and side gears.
Both front and rear differentials follow the same design, and all of these are lubricated with thick gear oil. The basic purpose is to transfer power while allowing the two drive wheels of an axle to turn at different rates, as is necessary when steering around corners.
What this also allows is for one wheel to spin independently of another, which is a great disadvantage in slippery terrain or snowy roads.
If one wheel loses traction, all engine power is sent to that wheel. For that reason, limited slip differentials were developed, which still allow driven wheels to turn at different rates but limit the amount so that some power will always be applied to both wheels. This is more often used on all-terrain four-wheel-drive vehicles.
The differential fluid in either design is subject to age and wear and has regular replacement intervals. These are very dependent on how a vehicle is used.
The most common service intervals are from 30,000 to 60,000 miles. The shorter period is for a vehicle that is used for towing, which generates a lot more heat and wear in the differential.
The longer period is for normal use. Sometimes, the interval is much longer, as wear depends not just on how a vehicle is used, but the use it is designed for.
A differential designed and built to tow 12,000 pounds, for instance, sees little wear or stress if it isn’t actually used to tow anything.
It is also fairly common to have a recommendation for the first differential fluid change that is much shorter, such as at 15,000 miles. That recognises the break-in period of the differential when the new steel-to-steel running gears bed into each other and generate more heat and waste metal particles than they do when they are well-mated surfaces that are polished from initial wear.
Different Kinds of Fluids Used
Most gear oils have a specific weight rating, such as 80w, or 80w-90, or 75w-140. The W signifies the tested viscosity rating of the gear oil in winter conditions, while the second number (if present) represents the viscosity in warm weather.
This is similar to how motor oil is rated, but the method and standards are different. For instance, 15w-40 motor oil, is pretty similar in consistency to 75w-90 gear oil. Sticking with the manufacturer’s recommendation is always the safest way.
One other common option that comes up is synthetic versus conventional oil. Both are commonly available with the same weight ratings.
Other than cost, the general consensus is that synthetic is better. Generally, they have a lower friction coefficient (and so gears run cooler), better sliding characteristics, and are less affected by temperature changes.
The same as with motor oil, it all comes from the same place. However, the refining method is more involved and precise, and so, the cost is greater.
On a vehicle that is under heavy use, a synthetic is probably a good choice and should allow a more relaxed service schedule. The advantage is much less in normal or light use and in mild climates.