Don't Use Gear Oil By Eldon McFarling Here we have a very confusing subject. There is much more to selecting the proper lubricant for use in manual transmissions & transfer cases than just the "weight" of the oil, or the convenience of having a single lubricant for everything. Gear oils of the multipurpose type (GL-5), and Mild EP gear lubes (GL-4), contain Extreme Pressure, or "EP" additives that are not suitable, or required in manual transmissions, and transfer cases. Straight Mineral gear oil (GL-1), with rust, and oxidation additives (R&O type) is acceptable for use in most manual transmissions, but heavy duty engine oil is the PREFERRED lubricant recommended by most manual transmission & transfer case manufacturers. one of several benefits from using engine oil is that the detergent additives in the oil keep the inside of the transmission clean. Manual transmissions, and transfer cases require lighter oil for lubrication because they have less clearance between major parts, such as synchronizers, gears & shafts, etc. Heavier gear oil cannot provide adequate lubrication between gears and shafts since it will not flow thru small passages, or tight clearances. Gear oils can also make shifting difficult, or impossible in cold temperatures. Gear oils with EP additives are especially bad when used in transmissions because the EP additives can plug passages that the lubricant has to flow thru, and can also build up on other parts such as synchronizers, and oil seals, causing early failure. Below are the Dana / Spicer Transmission Lubrication recommendations, which includes Dana / Spicer main & auxiliary transmissions, and transfer cases. The PREFERRED lubricant is heavy duty engine oil, with a viscosity grade of SAE 30, 40, or 50 for temps above 0 degrees f. If the proper grade, and type of engine oil is not available, straight mineral gear oil can be used. Multi-purpose, Mild EP, or any other gear oils that contain EP additives SHOULD NOT BE USED in manual transmissions, and transfer cases. "But my IH manual sez"... The IH lubrication section from the service manual is very confusing, and difficult to understand. It helps to consider that the same section is used in all service publications for the various models of trucks that IH produced, which would cover everything from a early Scout, pickup, or Travellall, to the largest trucks that the IH truck division made. The recommendation for all transfer cases except code 13143 (IH single speed) is straight mineral oil, which is acceptable according to the Dana / Spicer recommendations, but under the "Spicer" listing in the same service manual section, engine oil is specified, the same as the Dana / Spicer bulletin calls for. The reason that mineral oil is specified for transfer case lubrication is that larger IH trucks, like 6X6 off road dump trucks for example, used transfer cases that require mineral oil. The consolidation of the lubrication recommendations into a single section for the entire truck division makes it just plain hard to understand, and also difficult to be precise for each individual vehicle. I always recommend following the information available from the manufacturer of the particular component being used. Where to get good quality lubricants will vary depending on location, but suitable transmission lubricants can be found at places where heavy duty oils & lubricants are sold such as transmission & driveline specialty shops, fleet oil & lubricant suppliers, farm stores, truck stops, truck dealers, and truck repair shops. Some transmission manufacturers have developed specially formulated synthetic oils for use in manual transmissions, and transfer cases. Availability of heavy duty engine oil SAE 50 is very good, and it is inexpensive. Availability of straight mineral gear oil (GL-1) is not as good, and in some areas it may not be available at all. To sum this up, EP type gear oils SHOULD NOT be used in manual transmissions, or transfer cases. Gear oils containing EP additives are recommended for all driving axles however. Both GL-4, and GL-5 gear oils are acceptable for Dana / Spicer, and other makes of axles used in IH Scouts, and Light Line vehicles. There are certain applications that do require different, or unusual fluids, and the manufacturers recommendations should always be closely followed. This is especially true for late model trans-axles, and transmissions. Some transmissions use a facing material on the synchronizers much like the clutch plate material that is used in an automatic transmission, if the wrong lubricant is used the transmission may not shift properly as you experienced. For this discussion I would like to focus on what is required for transmissions & transfer cases used in IH Light Line vehicles, and Scouts built from 1960 thru 1980. There are 6 API classifications that are used for designating the various types of gear lubes. These classifications were adopted in the mid 1960's. The classifications are GL-1 thru GL-6, the major difference in these is the amount of "EP", or Extreme Pressure additives used for each one. GL-1 = Straight Mineral Gear Oil GL-2 = Very Mild EP Gear Oil GL-3 = Mild EP Gear Oil GL-4 = Medium EP Gear Oil GL-5 = High EP Gear Oil GL-6 = Very High EP Gear Oil The classifications most commonly used are GL-1, GL-4 & GL-5. The only classification that does not contain any EP additives is GL-1. Extreme Pressure additives are necessary when gear teeth are heavily loaded, like they are in driving axles. Engine torque is multiplied by the transmission, and transfer case, so the load on the gear teeth in the axle is very high. EP additives make the gear oil very sticky, and a coating from the additive is formed on the tooth surface that prevents metal to metal contact under high tooth load conditions. Transmissions & Transfer cases do not have the high loads on the gear teeth, most have helical type gearing which has several gear teeth meshed at the same time, and since the input torque is less the pressure applied to the gear teeth is not as high, and is carried by several teeth at the same time, so the Extreme Pressure additives are simply not required. In order for the oil to reach areas between gears, and shafts a lighter oil is also required. Another function of the lubricant is to carry heat away from gears, so an oil that flows easily is essential. Another consideration is the ability of the lubricant to flow at extremely cold temperatures. Gear oils can make shifting difficult in cold weather. Here's what I would recommend for axle lubricants in order of preference: Gear oil meeting GL-4 requirements, 80w90 Gear oil meeting GL-5 requirements, 85w140 Synthetic gear oils are also acceptable For limited slip differentials a friction modifier additive should be used For manual transmissions, and transfer cases: Dana / Spicer, New Process, Warner, IH T-34,T- 35, & T-36 Specially formulated SAE 50 transmission oil Heavy duty engine oil, SAE 50, or aircraft engine oil, SAE 50 Straight mineral gear oil, GL-1, SAE 80, or SAE 90 IH T-494, T-495, T-496: SAE 10-30, or SAE 10-40 multi-grade engine oil (no substitution) A zinc additive should be used to prevent excessive shift fork wear Synthetic oils are acceptable for all transmission oils My $.002, find a farm store, truck stop, or other oil supplier that has 50W engine oil, buy a 5 gallon pail so you have some on hand, then drain, and refill your transmission & transfer case annually. This is probably the best preventive maintenance that you can do.