85 Coupe GT Gearbox Oil

IronJaw · 27-09-22, 10:02 AM

Hi,
Forgive me if this has already been asked but I’m just wondering what the best oil to put in the gearbox on a coupe GT manual is? I’ll be changing it over the winter months before the car comes out next spring so just getting stuff in stock ready to do all the work.

Would good old EP 80/90 be ok or would I need something more specific?

The car drives ok, but 36 year old oil in a car that’s only done 78K miles will definitely want renewal.

Thanks

84CoupeGT · 27-09-22, 10:26 AM

Have been researching this for my 84 Coupe GT manual recently. According to the Haynes and various sources online, what you suggest (80w/90) GL4 gear oil is recommended (not GL5 as that isn't compatible with some components in these older gearboxes). I've seen Castrol recommended a couple of times, it's slightly more expensive than others but I'll go with that when I change mine.

IronJaw · 03-10-22, 05:49 AM

I have a Haynes manual here and for the most part I find it very good at not providing me with much information. I will get some fresh castrol 80/90 and some cheap 80/90 to flush the box out with just to give it the best chance.

I hate gearbox oil though, it’s smell is the worst.

84CoupeGT · 03-10-22, 09:00 AM

Quote:

Originally Posted by IronJaw

I have a Haynes manual here and for the most part I find it very good at not providing me with much information. I will get some fresh castrol 80/90 and some cheap 80/90 to flush the box out with just to give it the best chance.

I hate gearbox oil though, it’s smell is the worst.

Yes, apparently the Bentley manual is far more comprehensive, however as far as I know they are for US models only and are also rather expensive.

Not sure the flush is necessary unless the oil is really terrible, however can't harm of course if you're feeling "flush"

. Sorry that was terrible.

SwallownAmazon · 03-10-22, 10:50 AM

Quote:

Originally Posted by IronJaw

I hate gearbox oil though, it’s smell is the worst.

Nooooo.... It's lovely - almost as good as the smell from a teaspoon of Mobil 1 in the fuel tank. Just like Castrol R........

IronJaw · 09-10-22, 07:45 AM

Quote:

Originally Posted by 84CoupeGT

Not sure the flush is necessary unless the oil is really terrible, however can't harm of course if you're feeling "flush"

. Sorry that was terrible.

That did make me chuckle if I’m honest

But in terms of the flush I just want to cover all the bases. Given the amount of shite I’ve found in the other areas of the car I’m willing to bet the gearbox would appreciate a flush.

I do hate the smell though, it offends my nose. The smell pierces it’s way into your nostrils and sticks to the back of your throat. Yuk!

27-09-22, 10:26 AM	#2
84CoupeGT Grown up member Join Date: Nov 2020 Posts: 309	Have been researching this for my 84 Coupe GT manual recently. According to the Haynes and various sources online, what you suggest (80w/90) GL4 gear oil is recommended (not GL5 as that isn't compatible with some components in these older gearboxes). I've seen Castrol recommended a couple of times, it's slightly more expensive than others but I'll go with that when I change mine.

03-10-22, 05:49 AM	#3
IronJaw Grown up member Join Date: Nov 2016 Location: West Lancashire Posts: 168	I have a Haynes manual here and for the most part I find it very good at not providing me with much information. I will get some fresh castrol 80/90 and some cheap 80/90 to flush the box out with just to give it the best chance. I hate gearbox oil though, it’s smell is the worst.