UK Car Repair: TR4 Overdirve, limited slip differential, friction modifier

Question
80w 90 Gear oil, generic brand from auto parts store
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Followup To

Question -
Have a 1963 TR4 with A-Type overdirve. Recently been havings problems with overdrive cutting out while engaged and I lift of accelerator quickly. Tried spare solenoid (not new), did not help problem. Engages perfectly. I did rebuild transmission 2 years ago but left overdrive unit alone. I did adjust the operating lever according to Haynes manual.

Answer -
What type of oil have you been using in the gearbox?

John

Answer
Your gearbox is full synchro with bronze rings. You need to use 10w-40 or 20w-50. Overdrives do not like to function with heavy oil and you will gall and glaze the friction cone if you continue to use it. AMS oil makes terrific oil called synchromesh oil that is designed with transmissions in mind. If you cannot find it, use Mobil One 10W-40 and check to make sure you have no leaks.
Gear oil is only used in the TR3 gear box with non synchro first gear and steel synchro. I would use regular motor oil for around 50 miles to work loose all the gear lube before swapping that out to Mobil One. I suggest adding a small amount of limited slip differential friction modifier as well to get some grip back in the cone assembly. You may or may not have damaged the relief valve "hence the fading power" and if you start to feel a sensation not unlike a slipping clutch you will need to rebuild your overdrive.
But, try an oil change first to see if you have lucked out and get back to me if you have any problems.   

Regards,

John