My 2003 Disco manual specifies 75W80 synthetic gear oil. I use my Disco mainly for regular work commutes with the ocassional light off-roading (gravel or sand, but no boulder trekking).

I searched for the forum.... many posts recommend 85W140, but does not explain the reason for using this grade of oil.

Any reason for the 85W140 being preferred over the standard 75W90?

Is the former recommended for heavy off-road usage or for other reasons through proven experience of the forum members (e.g., 180-degree thermostat recommendation)?

Thanks for any insights!

 

Heavier weight provides more protection at higher temps and I personally have always gone with heavier weight as something ages and wears down. Heavier is said to possibly cause a minor drop in mpg, so you've kinda gotta pick your poison really. After doing it myself I'd go protection over mpg.

I recently replaced all the gear oil in both my 99 and 04 with 75w90 only because I had a ton of it left over from when we use to farm. The 04 is fine, because it only has 100,000 on the clock and is quiet and smooth. The 99 is well over 200,000mi and I've decided due to the new road noise caused by old worn gears and thin oil I'm gonna have to go heavier (85w140) in it.

 

Only use the 75 or 85/140 as already mentioned it is a far better oil and if was in production when our Rovers were that would have been the recommended weight.