Substitutes for Gear Oilby Dan Eash
When you add gear oil to your manual drive train, it performs four important functions: flushing away contaminants, lubricating moving parts, keeping them cool and protecting them from weather and corrosion. Your transmission or differential are constantly exposed to high pressures, which become greatest when you accelerate or slow down. Gear oils have additives to reduce these pressures, but many other lubricants don't. If you don't have access to gear oil, use a substitute that is safe to use under the right conditions.
Manual Transmission Fluid (MTF)
An obvious substitute for the American Petroleum Industry Gear Lubricant (API GL-4 or GL-5) typically used in your vehicle is an appropriate MTF for your make and model. Unlike GL-4 and GL-5 oils, which meet specific performance and viscosity requirements, MTFs can vary widely from one brand to the next. As such, you need one that comes recommended by your original equipment manufacturer (OEM). Your OEM will only endorse an MTF that has the right viscosity and performance for your drive train so it's always a safe substitute for the equivalent GL-4 or GL-5 oil.
Automatic Transmission Fluid (ATF)
While ATF's name alone may indicate an unsafe substitute for manual transmission gear oil, some exceptions apply. According to lubricant manufacturer Opie Oils, "Certain designs do specify the use of an ATF in manual gearboxes, but they should only be used where it is clearly stated by the manufacturer." One example, the fifth-generation Toyota Celica manual transmission, has an S-53 performance option that has tight internal clearances. These clearances prevent gear oil from reaching some areas; a lower-viscosity ATF can provide lubrication where needed.
You wonder whether you can get away with using motor oil in your gearbox; you wouldn't be the first with this idea. According to Opie Oils, "Engine oils can be used in certain gearboxes, in the past it was the norm to do just that." One example is the classic MGB sports car. The "Haynes MGB Owners Workshop Manual" recommends 20W50 oil for both the engine and transmission with changes every 3,000 miles for the engine and every 12,000 miles for the transmission. Many of these classics have phosphor-bronze components that contain large levels of copper. The sulfur in gear oil additives attacks the copper and destroys the drive train's integrity. Motor oil doesn't have this additive, so you can use it safely. If you use a shear stable oil (one that has a long service life), it will last longer.
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