As the title says. 

The manuals recommend 75w/90 Fully Synthetic oil. This is quite expensive. The specs for these oils say "For use in manual and "dry clutch" transmissions where API GL-4 lubricants are required."

I have some Shell 80w/90 GL-4 Mineral. Is there any reason not to use this oil? This car does have over 150,000 miles on the clock so the box will be worn by now. 

 

Can anyone answer this?

My only comment is: does it need to be changed as if it's EP  it will cause further chemical machining.

Otherwise, the spec looks close but I wouldn't take any advice as definitive  from random blokes (like me) or gals on a forum 😁

I'm going to read this:
https://www.castrol.com/en_us/united-states/home/products/commercial-vehicle/understanding-the-differences-in-gear-oil-specs.html

And:
https://www.honestjohn.co.uk/forum/post/index.htm?t=59535

Thanks for the reply. The oil will have to be drained as the box will be coming out to change the clutch. 

The same place that will be doing the work say that mineral oil can cause stiff shifting.

As the box has no drain plug, they say it is best to put the recommended oil in there. It can't be easily drained once filled. 

 

You can not mix Fully Synthetic and Mineral Oil.

Also you will never be able to get all of the old oil out. The seals in the gearbox will be impregnated with the original synthetic oil. Changing to mineral oil will not end well, and should be avoided.