GL4 or GL5 inthe differential

Which one for the XKs final drive(without LS)?

I use GL4 in all my classic diff units. Rightly or wrongly, I have been told by the restorers who put together the Cobra’s aluminium T10 gearbox, the diff unit, my XK’s diff, Land Rover diffs etc. that using GL5 risks damage to the yellow metal components. I have seen a few comments saying it’s safe, but the guys who did the work said no, GL4 obviously works, so GL4 it is. For me.

All GL5 gear oils satisfy the engineering requirements of GL4. As such, GL5 can be used in any hypoid axle that would specify GL4. This does not categorically mean that either grade is also suitable for manual transmissions. GL5 has about double the the sulphur compounds found in GL4. These sulfur compounds increase the extreme pressure lubrication properties, so in severe conditions, GL5 provides more protection. However, the sulfur additives can be corrosive to brass. Since there are generally no brass parts in hypoid axles, that is not a problem. However, it can become an issue if it is used in a manual transmission where there are brass synchronizers or blocking rings, and can lead to early excessive wear on those parts.