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MG MGA - GL4 & GL5

Have I done something stupid? I decided to change the oil in the back axle and also top up the steering rack. I went to Halfords to by 2 bottles of their own brand EP80W/90 GL-4. Without realising, because the bottles looked identical, the second bottle was in fact GL-5.
I have found comments on the internet suggesting GL4 & GL5 can be mixed and that modern GL5 no longer damages "yellow" metals (don't know if there is yellow metal in back axle or steering rack)
Is that OK do you think or should I drain both and start again? I only do a very low mileage.
Thanks
Graham M V

Not to worry, should be fine in the differential and steering rack. Just don't put EP oil in the gearbox.
Barney Gaylord

Thanks Barney. Appreciate your help
Graham M V

GL5 is a different spec, and is not 'backwards compatible' with GL4.GL5 gear oil is a lubricant with very high extreme pressure performance needed for Hypoid gears where sliding and rolling contacts operate under very high loadings. This is achieved by incorporating high concentration of sulfurized additives. The downside to such high sulfur in oil is it tends to corrode yellow metals, so can ruin parts like brass synchronizers and some bushings. So any equipment needing GL5 performance is designed taking this limitation of the lubricant into account.GL4 oils are much more friendly to yellow metals, and so are used in equipment where such high load sliding-rolling contacts aren't present. The designers of such equipment aren't as limited in the use of brass parts. So the bottom line is that if your equipment calls for GL4 it probably doesn't need GL5 performance, and you may be asking for trouble down the line if you use a lube it wasn't designed for.
GL5 protects steel/iron parts better than GL4. BUT the things that make it better in one application can cause problems in another. Our differentials can use either. Both are wrong for our transmissions.

R J Brown

Thanks RJB. I only put it in the differential and steering rack, so am relieved you agree with Barney that the differential should be OK with GL5. Do you also agree that the steering rack is ok with it?
Graham M V

Will it have any effect on the thrust washers in the differential?
Dave O'Neill 2

Dave -
No, and it is essential to use EP for the hypoid gears. It has been used since the beginning of such gears and the bronze TW in the diff do not suffer.

FRM
FR Millmore

steering rack is ok.
R J Brown

Halfords ? I didn't think anybody went there for serious car stuff these days ( apart from possibly U-Pol Acid#8, for some reason they are mentioned as the cheapest stockist ).

Wilkinson hardware has Comma EP80/90 GL4 at much more sensible prices.
J N Gibson

This thread was discussed between 07/05/2011 and 09/05/2011

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