Hi,
I would recommend you check several things related to your clutch, just to eliminate concerns and possibly head off any problems that might be starting to develop. Clutch problems can cause the symptoms you have noted and are all too common on TR6. There are a variety of fixes that I won't try to detail here, but especially check that there is no slop in the clutch linkage (wear in the fork or clevis pin, free movement in the actuating lever that might indicate a snapped off taper pin inside the bellhousing) and that the slave cylinder is moving the pushrod a full 5/8" stroke. Also check that the clevis pin is fitted to the middle hole in the lever, *not* the top or bottom. All TRs use the middle hole... using the other holes can either over-extend or under-disengage the clutch.
Check for end play in crankshaft (thrust washer wear) with a dial micrometer on the front pulley/hub. On the 6-cylinder engines, this is another common thing that can effect the clutch and gearbox.
With these other things eliminated as possibilities, I generally agree with the previous responses:
No, transmission fluids are far too light for TR gearboxes.
Regarding non-OD gearboxes:
Original spec was 90w GL4 gear oil (never use *any* GL5, which contains sulpher compounds and will eventually damage brass parts in the g'box, such as the synchro rings). This is also specified for the differential.
80w90 or 85w95 is more common today and might be what you have to use. However, it may be too heavy when cold and can give symptoms like you describe.
20w50 *racing* motor oil can be used instead and is just slightly lighter than 85w95 gear oil.
Racing oil is non-detergent, which is what you want. Don't use any standard types of engine oil that contain detergent and especially no "high mileage" oils that contain lots and lots of detergent additives. The gearbox doesn't have combustion gases in it, like the engine does, so doesn't need any detergents. In fact, detergents can do harm to the gearbox by foaming and reducing effective lubrication.
If freshly rebuilt or just if it still seems a bit too heavy, especially in colder weather, don't be afraid to try straight 40w *racing* motor oil. Some go as light as 30w.
Regarding an OD/gearbox with non-synthetic oil:
Everything the same, except in a freshly rebuilt OD 30w racing motor oil is probably best (recommended by John Esposito at Quantum Mechanics, who has rebuilt many TR gearboxes and overdrives). 40w racing motor oil might be used, in part or mixed with 30w, if the gearbox has some miles on it.
Original specification even for OD gearboxes was 90w gear oil. In really hot weather, with high mileage on the OD, or in hard driving conditions, heavy gear oil or 20w50 might be used. But some have reported ODs locking up when hot, if heavier oils like these are used. Tests done by John Esposito showed internal OD pressures getting way too high with heavier oils. AFAIK, all his testing was done on fresh ODs, so I don't rule out using 40w and even 20w50 in an OD with some time and miles on it.
Regarding synthetic oils in a TR gearbox:
A number of TR owners have reported problems with gearboxes popping out of gear, after switching to various synthetic oils. I haven't tried them, but have heard certain Redline gear oils are an exception and might work fine, because they are specifically formulated to not be as "slippery" as many other synthetics. There may be other usable synthetics out there, but I'm not aware of any brand names. If you choose to use synthetic gear oil and the gearbox starts popping out of gear, most likely no harm will be done and the problem is usually solved by simply switching back to non-synthetic oil.
Regarding synthetic oil in an OD gearbox:
I would be a little concerned about using any synthetic in an OD gearbox. The reason is I accidentally ruined an oil-bathed clutch in a motorcycle when I mistakenly poured in a quart of synthetic oil. Immediately the clutch started slipping and had to be replaced.
Guess, what? The TR OD units have an oil-bathed clutch in them! It might or might not be a problem. I don't know for certain one way or the other. I just don't see any reason to take a chance. ODs are costly to have rebuilt!
Hope this helps.
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