At 11:18 AM 12/3/04 -0500, Paul Hanley wrote:
".... the basic design and parts are the same as the early MGB. .... John Twist of University Motors, Ltd endorses Castrol 20w50 for all MGB gearboxes. Therefore, I use Castrol 20w50 in the Twin Cam gearbox."

Ah, professional endorsements are a noble cause (usually), and I happen to agree with this one. But it is important to bear in mind that such endorsement should be founded in long term experience (which JT certainly has). I have been using 20W50 oil in my MGA gearbox(es) for decades. I have seen the before, during and after results, and I can tell you there is no problem with this.

Original specification was indeed straight 30 weight nondetergent oil. That's partly (mostly) because it is what was generally available back in the day. Concerning modern oil(s), any multiweight oil that spans the 30 weight number will protect the mechanical parts as well (and in most cases better). So 10W30, 10W40, and 20W50 will all serve the purpose. Using low viscosity oil in severely cold weather is not so important in the gearbox as it is in the engine, so you might run 20W50 in the gearbox all year round, and it may help your gearbox survive a little longer than lighter grades.

Do NOT use EP-80W90 hypoid gear oil in the transmission of the MGA or early MGB with three syncro gearbox. The sulfur additives are required for Extreme Pressure on the oil at point contact in the teeth of the hypoid gears in the ring and pinion gear set in the rear axle. But the 3-synchro gearbox has brass synchronizer rings and some bronze bushings for 2nd and 3rd gears on the mainshaft, and the sulfur additives in EP oil will eat the brass parts. It may be okay to use EP oil in the later MGB 4-syncro gearbox, which has steel synchronizer rings, but probably not necessary. MGB gearbox oil was spec'd as EP gear oil for a short time, but then the spec was changed back to regular engine oil.

The other issue is detergent. The first point to make here is, contrary to some beliefs, detergent oil does not foam up in a gearbox and will not cause problems with bubbles.

In general, you can think of "detergent" in oil similar to detergent in the kitchen sink. It tends to keep "dirt" particles in suspension in the liquid. For an engine this helps to move the larger dirt particles along with the oil until it can get trapped and filtered out in the oil filter. It also allows the smaller particles (which could pass through the filter) to remain in suspension and to be drained out along with an oil change. This helps to keep the engine clean and prevent the build up of sludge in the engine.

For a gearbox detergent oil does the same thing, although it's not nearly as important as in the engine. The gearbox has no oil filter (except a screen in an overdrive unit or an automatic box), so you might think that keeping dirt in suspension might be bad for wear on the mechanical parts. But that doesn't seem to be that case, at least not if you change the oil occasionally.

With nondetergent oil in the gearbox, any dirt or wear particles are likely to settle out in the bottom of the gear case, especially when the car is parked. Even as much as I drive my MG, I have yet to know any MG that is not sitting silent more than 90% of the time. Anyone who has opened an old gearbox may have found a noticeable accumulation of "mud" in the bottom of the case, sometimes worse than others. In fact if you never change oil in the gearbox, non detergent oil may be better for separating out the dirt and leaving it at the bottom of the case where it might do less harm. I do not personally subscribe to nor recommend that habit.

With detergent oil in the gearbox, the smaller and lighter "dirt" particles (the ones that don't cause any noticeable harm) will remain in suspension for a long time, even when parked. As with an engine, this tends to wash the innards and keep it clean, and the dirt can be drained out with the oil during an oil change, which should prolong the life of the gearbox in the long run. Larger and heavier particles, such as visible metal bits that wear away during normal operation of the gearbox, will settle out in the bottom of the case regardless. Even with detergent oil, the visible metal bits are heavy enough to drop out of suspension and will not be carried around with the oil. These will still accumulate in the bottom of the box, generally not causing any additional harm to the moving parts once they have first escaped the mechanism. Some of this can be drained out with an oil change, especially if it's drained immediately after a long run with hot circulation. But some of it will always remain behind, so next time you open the box you will still find at least a small trace of some silver and/or yellow particles in the box. But with detergent oil you are much more likely to find a generally clean gearbox and very unlikely to find any significant accumulation of mud in the bottom.

".... Another product often endorsed on the MGB/MGA bbs is Redline but I haven't tried it."

I have tried it. I just drained the Redline MTL out of my gearbox after a two year trial run and refilled once again with 20W50. I have a couple of personal observations, and I'm not doing back flips over it, but I'm also not sure my single case should translate to be binding on the world at large.

First, I keep a gearbox in good working condition at all times. When it may be approaching high mileage, at the first sign of any problem it's out for a rebuild. So I can't attest to how well the Redline might work in an old worn out gearbox. My best guess is that worn out is worn out, and no magic elixir will fix it (or certainly not for long).

Second, the Redline product is prohibitively expensive. Well it's not so expensive that we can't afford to use it to pamper our pet car if we want to spend the money, but it's a luxury item that may have little or no economic justification. If you use the stuff regularly and change it periodically as prescribed in the shop manual, the increased cost of the Redline oil will exceed the cost of rebuilding a gearbox. And it doesn't matter if it could somehow magically double the life of the gearbox, it's still not economically justified.

Then it gets down to a discussion of whether Redline may bring some appreciable benefit other than increased life of the gearbox. Some people say it makes the gearbox shift better. I have not had that experience. In fact I did notice a difference (a very slight difference), and I personally prefer the way it shifts with 20W50 oil over the Redline results. Surprise.

After installing Redline MTL in my gearbox I immediately noticed that it would "grind the gears" slightly when shifting, which it did not do with 20W50 oil. After some experimenting (fiddling about) with various shifting techniques, I found it would work better when I forced the shift a little quicker. I needed to use a firm hand on the shifter rather than being gentle with my finger tips. In fact it took about a week of driving to intentionally train myself to do this habitually, as it is counter-intuitive to ham fist the gearbox on a regular basis for normal street driving. Otherwise the stuff works okay, aside from the fact that I could never determine any functional advantage. Perhaps some competition folks might find it better for speed shifting, but I've never had any problem with 20W50 in that respect either (and I do plenty of speed shifting in competitive autocross).

After a two year trial run, a few weeks ago I drained out the Redline and once again installed 20W50 oil in the gearbox. The small difference in the way it shifts was again immediately noticeable, and I can now go back to using gentle fingertips on the shifter for normal daily driving. Knowing that one of the most wear prone points of the MGA and early MGB gearbox are the three little ears on the brass synchronizer ring, I think I prefer being able to use the lighter touch for daily driving.

If the Redline product wasn't so darn expensive I might have continued to use it for a more extended trial run to attempt to evaluate any possible difference in mechanical wear. But frankly I can't afford it without having some other tangible benefit to justify the cost.

$.02,
Barney Gaylord