About to change the gearbox and diff oils in our 1.6 MK1 and was wondering what make and viscosity does everyone recommend ? I’m torn between a cheap old Halfords own or spending a liittle more on a good name brand - MX5 Parts sell a Mobil1 oil for a few quid more. Mobil only do the oil in 75w90 but other manufactureres still do 80w90, which I think was the original weight oil (someone correct me if I’m wrong!).
Also wanting to replace the turret oil but can’t find a capacity for it anywhere, does anyone know if there’ll be enough left out of 3 litres of gear oil to do it?
I bought 2 bottles of Redline MT-90 for the gearbox and one bottle of Redline 75/90w for the diff. there was some left in the gearbox oil for the turret. Got it from JJC if that helps
Purely imho but if you’re shopping
at Halfords I’d get 3L Castrol Syntrans 75W90 GL4 Transmission Fluid
for the box and 1L Castrol Axle EPX 80W90 GL5 for the diff.
The box can use GL5 but is not
needed. GL4 works better with synchro hubs and should improve the
action. I tried Mobil SHC in mine and it gave a very poor shift. I
use the Castrol now which is much better but your choice of course.
Be sure you can get the 14mm square taper plug out the side of the
box before draining it or you may be stranded. An oil gun/syringe
such as from Machine Mart for £8 is worth its weight in gold.
Hadn’t realised that the gearbox and diff needed different weight oils, I was all set to get the same one for both, now I’m really spoilt for chooice!
Good to know that there’ll be enough left over for the turret though!
I’ve got a load of washers for the sump plug, am I right in thinking the diff and gearbox drain plug washers are the same size as the sump and that these need changing as you would when changing the engine oil?
Ooh, I’m getting really confused now lol! Sorry if I’m asking dumb questions but…
I like the sound of the Castrol Syntrans, but I’m thinking that the Axle EPX they do might be a bit overkill for me. Would I be ok using Syntrans 75w90 in both the gearbox, diff and turret? I’m not going on track, I kinda drive it like Miss Daisy (as my hubby keeps reminding me!), and we’ve not got any performance bits on the car, it’s a pretty bog standard 1.6.
I think whats confused me is the bottles generally have “gear oil” on them giving me the impression that they’re one and the same for gearboxes and diffs. Also MX5 Parts sell the same grade Mobil1 for both
Yes Redline is good too, used it in our Golf Gti a long time ago, but you will still need 2 oil types to get the benefits doing an MX5.
The gearbox can physically use GL5 but really needs GL4 spec for the best gear change (applies to Redline or Castrol). This gives ample protection for the gears but is formulated as a manual transmission fluid (not axle oil) to allow the synchro hubs to clear it and work properly.
The diff needs GL5 spec axle oil for protection as it is hypoid gears.
The Mobil SHC that MX5parts sells is an ultra heavy duty GL5 truck axle oil. See all the trucks listed on the data sheet, the viscosity sits at the top end of specification, and it’s GL5, which altogether means that when I tried it in our car a couple of winters ago I couldn’t shift gear at low temps. A waste of £40 and an hour messing around but it may be ok in summer.
Im new here, and have been reading this topic with interest - sold on the idea of going the castrol oil route - (as Im just about to change diff and gearbox oil) I have mk2.5 and havent seen anything about a turret in the gearbox and replacing oil in it, in the Haynes manaul I have. Can someone enlighten me as to the tuuret its fitmanet etc, and how you change the oil in it.
so plenty after 3l as the box holds 2 
