I personally don't think it matters which one you choose. They all work ok in my book. Even with regular gear lube, you can get way more miles out of an axle assembly if you change it often enough, which most people don't. My chevy truck has 246k miles on the original rear end with regular lube and it's not falling apart. Just choose a brand of Synthetic that you can get consistantly so you don't end up mixing. Change it often enough ..and that depends on driving conditions as well as age/miles. Mobil 1 seems expensive from what I've seen, Royal Purple is hard to get ..and last time I got a price it was kinda high. Valvoline Synthetic is one good choice B/C you can get it at just about any NAPA and price is reasonable. I have used it vehicles for years. Couple yrs ago I since switched over to Walmart fully synthetic cause it's 1/2 price. Seems to work just fine. I can't remember who supplies it, but they told me at the time.. (subject to change too)..

Is one brand 5% ..maybe 10% better than the other? Who knows.. maybe, but probably not. Does it really matter on my Disco that already has 135k mi? Almost for sure, "NO". I think you will be fine with any choice of the products you listed.

 

Can i replace my gear oil on a disco 1 with 80w 90 conventional or does it have to be synthetic? Thx.

 

You can, but most recommend synthetic.

 

Conventional is fine, it's all I've ever used in Rover diffs for 40 years. Make sure to check the level on schedule and change it on schedule (or more frequently if you off-road in water) and you'll be fine.

I would suggest 85w140 however.

 

Any gear oil, as long as (GL5), is better than running it too long without change,
If you can't change it often enough, synthetic will last better In long change intervals.

 

OK great thank you so much.

 

By the way do i need any limited slip differential additive. I'm planning on doing both differentials and the transfer case. Thanks.

 

No.
The Land Rover limited slip diff that used clutches was discontinued in the 60's, and wouldn't fit the Disco half shafts anyway.

 

Oh OK great that save me a buck. by the way I bought the 85w 140 gear oil and drained and started filling the transfer case with it. It seems very thick. I live in south Florida where its in the high 80s most of the year. I know you recommend it but would it be OK to use for the transfer case and both the front and rear differentials? Thanks. It's a 1999 disco 1 by the way

 

I use 85w140 wherever gear oil is called for.