High-grade gear oil must provide both lubrication and cooling to geared systems while simultaneously carrying away wear debris that accumulates at contact zones. Furthermore, it must withstand extreme pressures and temperatures without breaking down, in order to prevent friction, scuffing or other forms of damage that could ultimately lead to gear failure.
Automotive differentials often use hypoid gears which require gear oil with a concentration and additive concentration equivalent to GL-5 performance; most manual transmissions use helical gears which don’t necessitate such high performance levels. Gear oil also has its own distinct scent depending on its additive content concentration.
Table of Contents
1. How to Check the Level
Gear oil in a rear end provides essential lubrication to axle tubes and bearings, but if its levels drop too low, axles may grind causing an unpleasant jerking motion to develop. Simply topping up fluid should remedy this situation; however if your vehicle displays signs of transmission slippage or chain grindage it might be time to investigate other potential issues that require further investigation.
Gear oil’s lubricating properties are determined by both chemical and physical factors. Conventional oil typically includes detergents to trap dirt and debris in suspension so as to not scratch metal surfaces inside of the gearbox, though these wear out over time. In addition, corrosion inhibitors help prevent rust and oxidation as well as anti-foaming agents to keep foaming at bay when the gearbox spins through, although even these measures have their limits and should eventually be replaced as well.
Many cars feature a dipstick located at the front of their gearbox that displays recommended oil levels. When this occurs, take care to remove and wipe clean before reinserting it to check on its status. If the oil level isn’t correct, if necessary use a syringe to draw out some old fluid before refilling to the proper mark with engine or hypoid oil (as per vehicle specifications).
Other gear boxes feature a filler plug and service port to make level monitoring easy. Just unscrew the plug, clean off any debris around it and open it; look for fluid; gear oil should reach to the bottom of its filler hole – if not top it up until gear oil starts flowing out through its service port; alternatively you could insert your finger or piece of wire down the filler hole to feel for signs of fluid.
2. Filling the Differential
No matter whether your vehicle is rear, front, or all-wheel-drive, its differential relies on differential oil to evenly distribute engine power between axles. Without it, parts could wear out or become damaged quickly resulting in your vehicle not performing optimally.
Draining and refilling the differential requires disassembling an attachment usually bolted in place (see image below). As this task can become messy, ensure there is plenty of floor space beneath your differential and wear clothing suitable for dirty work before you begin. A jack and jack stands may also come in handy to lift it off the ground for this process.
First, remove the differential’s drain plug. Drain its gear oil; if its color looks discolored or contains metal shavings, change it immediately. Additionally, if your differential features a magnetic drain plug that requires cleaning before being reinstalled, remember to replace and clean its crush washer before reinserting the plug.
Once all the old oil has been drained off, reinstall the drain plug by starting it by hand to avoid cross-threading and tighten it according to manufacturer specifications. Use new gaskets when installing new covers; when reassembling differential covers apply gasket sealer as needed.
Add new gear oil to the differential, using only what has been recommended by your automaker (this information can be found either online or in its owner’s manual).
Start with a funnel and fill to within an inch of the fill hole with oil, using a funnel as necessary. When the hole can no longer be seen, reinstall the fill plug and reassemble your differential.
If you need assistance selecting the appropriate gear oil for your vehicle, Blauparts or RAVENOL offer professional guidance to assist in making an informed choice from their extensive selection. *See Blauparts disclaimer and terms of service for more details.
3. Removing the Filler Plug
Your differential likely contains metal shavings from oil changes gone awry; to clean it properly it would be wise to use degreaser and shop towels containing antistatic agents to wipe down its mating surfaces – housing cover and gear oil fill plug – before running it through a magnet to collect any fine metal particles that might remain.
Rear diffs typically require one or two quarts of appropriate gear oil, depending on their type. If adding all four quarts at once, however, then the filler plug must be opened prior to filling up your reservoir.
An appropriate wrench, usually metric, will be included with your axle service kit to remove the plug; alternatively a big box wrench should do. Before reinstalling the plug it is important to note that its threads will likely have become filthy from scrubbing; these dirty threads must be cleaned off with anti seize before reassembling as otherwise your new gear oil could become polluted with debris from this process and possibly lead to premature wear and tear.
4. Reinstalling the Filler Plug
To avoid weeping oil-fill-plug syndrome, Taco Moto Co has developed a metal bayonet fill plug which fits securely into the differential cover (and includes a tab to stop it turning) with an O-Ring for high temperature applications. They offer this CNC machined from 6061 billet aluminum so it will hold up under the extreme conditions encountered while diff oil filling is being done.
Before beginning refilling, it is a good idea to wipe down both the housing and wet side of the cover with degreaser and use lint-free shop towels to wipe down both surfaces. Furthermore, run a magnet around the cover in order to detect any metal shavings present and pick them up as you go along.
Rear ends typically require between one and two quarts of gear oil for proper functioning. We suggest the XPS 75W140 synthetic gear oil; it provides protection in temperatures as low as 75F while still protecting at higher temps (140).
Loosen and remove the top-front differential drain plug before draining old oil into a catch can. Replace both drain and fill plugs as necessary with new crush washers; fill your diff with fresh gear oil using a funnel, replacing all required plugs as necessary with new crush washers (it is preferable to tighten by hand so as to not cross thread), tighten all connections to spec and dispose of old oil responsibly (this includes where to dispose it safely); also consider changing out its washer since accumulated debris may clog it over time and accumulate on its threaded threaded connections! Change out its drain plug’s washer because its rubber washer becomes dirty quickly due to being infiltrated by sticky build-up on its connection point with its counterpart located inside its reservoir!