The Service Manual for my 1968 Series 1.5 E-Type calls for Castrol Hypoy in the differential. Castrol only makes Hypoy with the newer GL-5 specifications (versus the older GL-4). Can I use gear lubricants with the GL-5 specification? If so, do I also need any additives for the limited slip differential?
All the seals and O-Rings have been replaced with modern materials, so I would suspect the newer additives in the GL-5 lubricants should not degrade them.