80/90 GL4 versus 80/90 GL5 - is it that much of a difference?

[VTRover]

Just wondering if it's so significant that I should only use GL4 80/90 versus GL5. What about synthetic gear oil that has something like 85/145 weight?

[Terrys]

Since you posted the question in the series section, I assume you're asking about using these gear oils in a series rear end. The GL5 oils are intended (as I understand it from other discussions) for use in late production diffs which are more likely to have Viton seals, as opposed to rubber seals in older Diffs. I can't say this with any authority, but what I have heard is that in older diffs, either is OK, but in new diffs fitted with Viton seals, only the latest formulations are reccomended. I think my gut tells me I would avoid using synthetics in a rubber sealed series diff.

[BackInA88]

Everything I have read on the different Land Rover forums (which is always hotly debated) is that GL5 is not good for the soft metals. It will eat them up over time. This would include the front diff if you have bronze bearing cages on the drive shaft support bearings, even though the oil seal should keep it way.
I checked into this when I rebuilt my trans and T-case and for that even on Mobils website they do not recommend their Mobil 1 gear lube for GL4 applications.
Redline and Royal Purple do make products that are GL4 compatable.
I am using Royal Purple Max-Gear 75-90 as it is available in quarts.

I have read that the GL5 spec says they are soft metal safe but the companies still say no on their sites??
I have put Mobil 1 in the rear as there is no soft metals.

There are also GL4 safe mineral oils still available I understand NAPA but I think it only comes in 5 gallon pails?
Here is some posts on the subject.

http://www.landroversonly.com/forums/showthread.php?t=17463

http://nhua.co.uk/smf_1-0-5/index.php?topic=12911.0

Have fun....

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