Ok,
So I am about to replace the Oil on my 2005 Manual 6-speed Gearbox & LSD diff
then I find out the oil is 75/90 for gearbox & 85/90 for diff....these are close grades & I ask if I can just buy a 4 litre of synthetic 75/90 & use it in both ??? will it be ok or do I have to spend more dam cash ? Like it's only a diff I considered the gearbox would be fussy..... Thoughts ???

 

Don't be such a tightwad. Get the right grade and type for each. Differentials can be just as fussy as gearboxes.

If you go synthetic, be careful which brand. There are posts here from folks who have had bad experience with at least one brand. I use genuine oil in mine, so I don't recall which synth brand(s) to avoid.

 

You have 3 basic proven options that are relatively easy to get:

1) Mazda OE oil for each
2) Redline MT-90 for the transmission, Redline 70w90 for the diff
3) Eunos oil for the transmission and one of the above for the diff

There have been numerous diffs and transmissions lost to people experiementing with other choices, so I'm not sure why anyone would pick something else. These 3 are proven to work, and work well.

Avoid anything at standard auto parts stores since they rarely carry anything that works. Royal Purple will destroy our syncros, for example.

 

Avoid any GL-5 (Royal Purple MaxGear) for the transmission though they should be fine for the diff.

 

If you use transmission fluid in the rear diff it will blow up, I've seen this happen first hand...

I use this in the transmission which is a GL4:
AMSOIL Manual Transmission & Transaxle Gear Lube 75W-90

And this in the diff which is a GL5:
AMSOIL Severe Gear® 75W-90

 

You could read the owners manual and do as it recommends. Check out Repco , Supercheap Auto dont carry Redline.
Arca, in Oz they refer to a manual trans as a gearbox, a transmission is an Automatic, dont confuse the man.

 

Ok Thx guys, looks like I'll have to go to Mazda today & pay the premium..... I was just hoping there was an all-in-one like I saw at Supercheap autos

 

The only catch here is that the '04 and '05 owner's manual says GL-4 and GL-5 are fine for the tranny while '06 and later say just GL-4.