A:     Hi Dan, the correct designation is actually EP90 HP (high pressure) hypoid gear oil. This is a fairly common gearbox oil and is the designated type for Land Rover and Leyland products. It's what I use in my B's differential. Transmissions use engine oil (straight SAE 50, which is getting harder to find, or 20W50 multigrade). I live in northern New Jersey and, while the weather may not be as extreme as Colorado, it gets pretty cold and hot here as well.

Below are some descriptions of the oils you were talking about and their properties. The 80W90 is closer the original EP90 and has lower pour point, so would well in colder conditions. It would be a good alternative if EP90 is not available. The 75W90 is describe as synthetic and has a higher viscosity index, so I would not advise it. G90/140 or 90W140 (not listed below, but questioned in your letter) is a "climbing" oil designed to cling to gears in extreme applications. I have used it in industrial reducers and, while very good, can be very dense (viscous), especially in cold weather. It can do more damage than help in most daily-use automotive applications.

In changing your differential oil, just make sure your 'whirr' is not just low oil related. A consistent whirr noise to me is the beginning of a bearing failure. Catch the oil and sift it through a paper or fine screen filter to see if there's any metal entrained. Also, there are these small fiber washer spacers that tend to wear, break and drop out. Their failure is usually characterized by a slight "clunk" when accelerating or slowing, but in the extreme can cause gear wear that will make more of a thrumming noise, but might actually be what you are hearing.

I hope this helps you. Let me know what you find. Good luck.

Art Isaacs

- HP gear drive is multipurpose gear oil containing extreme pressure additives for moderate service condition

- Meets API GL-4 , MIL – l – 2105 & EP type GL-4

Level of IS 1118

- Application areas are manual transmissions of automobiles requiring this type of extreme pressure oils

- Excellent thermal & chemical stability

- Protection against rust & corrosion

Ensuring longer trouble free protection

Kinematic viscosity, cst. @ 100°c 16.5 -17.3

Viscosity index, min 90

Flash point, (coc), °c, min 180

Pour point, °c, max - 9

Corrosion copper strip at 121 °c,

For 3 hours , max 2

Channel Point - 18

- Hp gear oil ep 80w – 90 is multigrade gear oil

Containing extreme pressure additives for moderate condition service

- Meets API GI-4 , mil – l – 2105 and EP type GL-4 level of IS 1118

- Application areas are synchromesh transmissions and manual transmissions of automobiles

- Excellent thermal & chemical stability

- Protection against rust & corrosion

Ensuring longer trouble free protection

Kinematic viscosity, cst. @ 100°c 13.5 -24.0

Viscosity index, min 90

Flash point, (coc), °c, min 165

Pour point, °c, max - 21