GL4 oil OK for rear diff?

I just did a rear diff oil change and only when I'd started it did I realise the oil I had was GL4 spec and not GL5 as specced in the FSM. I had to put it in to get home from the shop but should I:

Change it ASAP or leave it in it won't make any difference.

But if it does make a difference why does it. I spent an hour googling this afternoon to make sure it'd be OK but I didn't get much sense it was mainly the other way round what if I put GL5 in where GL4 should go.

Any techies help me out on this?

The GL5 lubricants have an Sulfur based additive which improves their performance under extreme pressures, but causes a corrosive affect on yellow metals (brass, copper and bronze)....
NOTE: Most synthetic GL5 lubricants donot use sulfur additives to meet extreme pressure requirements...

The GL4 lubricants may have some Sulfur based additives, but generally A LOT less.... GL4 lubricants are expected to pass the ASTM D130 (Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test) which it could not if too many sulfur additives were added....

In a nutshell.... The chemicals added to the GL5 lubricating oil formulas are designed for Extreme Pressure use (heavy duty) where the chemicals added GL4 are not.

yeah I got that much this afternoon too but will it hurt the diff if I use GL4 permanently?

As long as you use the synthetic lubricant form of GL4 and dont tow with your vehicle...; then using the present lubricant should be fine...
Just change back to what is supposed to be used during the next maintenance period.

No towing or offroad at the mo just a DD but I can't remember wether it was mineral or not. I'll change it sooner rather than later just to be safe.

Thanks for the replies.

Mike

I would change it myself. If you have the big syringe looking thing, you can just suck out the old and refill instead of popping the cover again.