- Requires API GL-4 oil.
- Do not use GL-5 oil. GL-5 oil will attack the brass synchros in the transmission and is not required for the differential. The SM (and DS) have a spiral bevel ring and pinion, not a hypoid ring and pinion, for which the EP formuation of a GL-5 oil is required. For further information see:
Automatic Transmission requires FORD TYPE "F" oil - do NOT use DEXRON or the transmission will fail prematurely.
Automatic Gearbox has a separate differential - use API GL-4 oil here.
- Red Line:
- regular climates : https://www.redlineoil.com/mtl-75w80-gl-4-gear-oil
- hot climates (or if synchros don't work well) : https://www.redlineoil.com/mt-90-75w90-gl-4-gear-oil
- AMSOIL: https://www.amsoil.com/p/manual-transmission-transaxle-gear-lube-75w-90-mtg/
- Valvoline: https://www.valvoline.com/our-products/grease-gear-oil/synchromesh-mtf
- Recommended by David Hume
Snake (transmission) Oil
The simplified version of how GL-5 gear oil works:
- Sulphur compounds etch the steel components.
- Residual sulfuric acid tends to form hydrates after removing oxides from the steel.
- EP additives fill the microscopic pits in the gear faces under pressure forming a malleable and slippery contact area.
The problem with GL-5 and yellow metals occurs when it is transmission unit only. There are two factors:
- The shearing forces are not adequate to embed the EP additives into the gear face.
- The free (residual) compounds can attack bronze or brass (copper alloys). There are, by the way, over 2000 documented copper alloys.
Oils are made from base stocks. They are then refined and given additives to modify their properties. GL-5 incompatibility with copper alloys is due to the extreme pressure (EP) wear additives. To gain the EP rating sulfur based additives (molybdenum disulphide) or zinc based additives (zinc phosphates) are used. The sulfur present in the oil is responsible for reacting with the copper alloy used to make the synchros. Keep in mind it also interacts with the steel (iron alloy) components.
GL-4 and GL-5 differ based on the amount of additives present. GL-4 has the same additives (surprise !!) but in a concertration about 50% of that in GL-5. Because the same additives are present in GL-4, they will eat the synchros as well. The service life of the gear train is beyond the corrosive effect timeframe. This is where the "GL-5 bad" assertion originates.
The yellow metal corrosion also occurs in unused vehicles. If the car is not driven and the lube does not reach operating temperature the residual acids will attack the synchros. If it is driven regularly it is not an issue.
Why? The gears in your transaxle reach higher temps and pressures than a typical transmission only box because the transmission and axle/diff share the same lube.
GL-5 in a regularly operated transaxle is not a problem. An additional measure of security can be added by using a copper insert in the transmission drain plug along with a magnet to trap the ferrous particles.
There is no harm in using the spec GL-4 but for a regularly driven transaxle car, GL-5 is not as big a problem as it has been portrayed. If it is a garage queen you can use a good quality marine 75-80W lower unit oil. It will not attack the metals and has excellent moisture control additives.
Many of the prohibitions or restrictions between GL-4 and GL-5 in a full synthetic do not apply. The modifiers use different chemistry to accomplish their mechanical behaviors and do not interact with metals in the same manner.