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How regular should I change the gearbox oil?


Jedi Trainee
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Gents, my gearbox incl. O/D unit was refurbished about 8 years ago. Til know I drove about 6.000 miles. While searching for a problem of the O/D activation I notices that there is "mud" in the oil filter of the O/D unit. Hence I changed the oil. My question is: how often should I change the gearbox oil? I thought that there is no need doing a regular oil change when drivin only a few miles per year.



Bob Hughes

Luke Skywalker
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If you have a hand book ( I have an original 100/6 book) it suggests that every 6,000 miles - 10,000 Km you should change the oil, and check it every 1,00 miles - 1.6 Km and top up as required. It would be a bit onerous if it was a daily driver but after 8 years it may well be worth it, I understand that the properties of the oil can deteriorate with time.


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I've used synthetic gear oil--Redline MT-90--for many years and miles. A couple years ago I pulled the gearbox and OD to fix a jumping-out-of-gear problem, and the box was clean and the gears perfect (the oil probably had 15-20K miles with that fill). The gearbox/OD oils don't get polluted by gas and combustion byproducts--or any dirt that gets past the air filters--and modern oils, especially synthetics, are much more resistant to shear and sludge than the waxy old dino oils prevalent when these cars were built (and the manuals written). Put in a good synthetic gear oil, or at least a good modern mineral gear oil and, out of an abundance of caution, leave it in for 10K miles or more (note a synthetic will likely leak a bit more). I use synthetic oil in both my Healeys' gearboxes and differentials, but use a good mineral oil--Valvoline VR1 20W-50--in the engines since our cars sit more than they move (I try to change them at least once/year, though I've been a bit remiss the last couple years).

In many/most/all? modern cars the differential, which experiences greater shear forces but also shouldn't get cruddy, has a 'lifetime' fill; i.e. it's not meant for replacement for the entire life of the car (which could be 200K miles or more).
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Whenever you get around to changing the gearbox oil take a couple of minutes and pour the used oil into another vessel in good sunlight. Seeing all the yellow stuff that had been floating around in suspension may lead you to shorten the change interval. Same goes with the diff oil.
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Non-detergent mineral oil was originally specified in the manuals. The theory, at least what I've heard, is that the tiny particles of ground metal will settle to the bottom of the case forming the 'mud,' whereas a modern detergent/dispersant oil would cause the particles to be suspended, as Michael attests. When I do change my gearbox oil, there will be tiny particles--judging by the color they are brass, likely from the shift forks wearing--that are visible in bright sunlight (kinda pretty actually). I was alarmed when I first saw this, but rubbing the oil between fingers I could tell the particles were extremely small, so small you could not feel them at all (note some greases; moly, lithium, etc. consist of finely ground metal thickening an oil). My take is the suspended particles are so small they do no damage--brass is a fairly soft metal--and will not cause any passages to be blocked. I don't know exactly how long I've been running synthetic gear oil in both my boxes, but it would be at least 75K miles, and I've had no problems that could be attributed to using synthetic. As a bonus, a good synthetic gear oil will give noticeably smoother shifts and, since the viscosity is more consistent an OD will engage slightly faster (esp. when hot). There is never any mud when using a proper synthetic oil; note I don't use synthetic oil in my engines, as I change the oil often relative to my daily drivers, and the synthetic will leak a bit more than conventional gear oils. I recently rebuilt my BJ8's box, and paid particular attention to surfaces and gaskets, and used a good sealant on the paper gaskets, and there are no significant leaks once I solved the activating cross shaft O-ring leaks.
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