Gear oil - GL4 or GL5

[olr159w]

Re-filling or topping up the gearbox or diff oil.

I used Castrol EPX 80W-90 in the diff. I'm about to top up the gearbox and OD. This Castrol product is GL5 spec

Viewpoints on GL5 spec vs GL4 spec?

There are comments in various places online saying a) it makes no difference or b) never use GL5 in the gearbox because it destroys syncro rings [and therefor by extension one might imagine OD contents]

[cliftyhanger]

Original spec was GL4.
Some GL5 attacks yellow metals when hot. Some don't (diffs and gearboxes get pretty hot in use it is where approx 10% of your engine power losses goes to, mainly ending up as heat)

Why take the risk? And does your diff contain any yellow metals? some have copper thrusts....

GL4 oils easily available. Or if you must use GL5, find one that is yellow metal safe.

Even my Type 9 Ford box uses GL4, and the best(probably) builder recommends a comma 75/90 GL4 spec oil.

[olr159w]

Fair point. The only issue is "GL4 oils readily available". Not really; at least not in the SAE 90 spec.

I can see some 75W-90 but not available nearby or Castrol VMX80 which seems to be 75W-80 or Nulon likewise a 75W-80. Looks like a trek to get the first one.

[soe8m]

GL5 spec is also too good for the gearbox. The synchro's are designed to cut the oil film from the gear cone. Then brake or accelerate the gear till both have the same speed and you can change gear. A GL5 spec oil is designed so sustain more friction than a GL4. The synchro's have to work harder or can't even cut the oilfilm. So it can happen that a gearbox does shift smoother with less quality oil than with a higher spec oil.

Jeroen

[olr159w]

What about the question of synthetic vs mineral?

Over here it seems like there are a couple of options for GL4 fully synth but only one I can identify for mineral-based - and the problem with that is that its a more obscure brand that doesn't seem to be well stocked.

[gmsclassics]

In NZ I have used Castrol VMX (GL4) in the gearbox (with overdrive) for almost 20 years and it was recommended by a local specialist rebuilder of gearboxes for classics. I use the same oil as you for the rear axle.

In the race car I use RedLine fully synthetic MT-90 (again GL4). No issues even when I am changing too fast for the synchro rings. A lot more expensive and probably an overkill for any road car.

Just in the same way I've always used Castrol GTX 20-50 mineral in the road Sprint and changed it annually, whereas the race car has Motul 300V 15-50 fully synthetic which is about five times the price.

I have always worked on the principle that you don't put synthetic in an engine simply by draining the mineral oil out. I've only ever used synthetic in a freshly rebuilt engine/ gearbox. That may help in your decision making.

Geoff

[olr159w]

I'd pretty much come around to Castrol VMX. The stuff I was trying to track down was this

http://www.hi-tecoils.com.au/wp-content ... p%2016.pdf

Purely because its weight spec is closer to the original. However its a large round trip to main supplier to get it vs VMX likely available 5mins around the corner.

I've just realised that from your picture you must have 3 Sprints then? Glutton for punishment........

I'm thinking of applying for a couple of jobs in NZ; if I happen to make it over there we'll have to catch up !!!

Thanks