MGB gearboxes (especially the early ones) aren't the most bullet proof of modern day designs. However, this doesn't mean they can't give reliable service over extended pbriods. As with any mechanical component, proper lubrication is critical.
A favorite topic of conversation among B owners is what sort of oil to put in these funny English gearboxes, and this is where the trouble starts. Everyone has a pet theory, even the factory books have been known to provide conflicting information. Whatever one might think, engine oil seems to work best, not 90wt. gear oil. Castrol recommend GP50 for 3 synchromesh gearboxes. The 2nd gear synchro will be slow when cold, but the gearbox will last longer.
Unfortunately, many people, upon hearing that the gearbox uses engine oil, mistakenly assume this means oil is fed to the gearbox from the engine. Not so. The gearbox has its own oil supply which leaks out on the driveway in the same manner as the engine's oil supply. A lot of "slow" MGB gearboxes are simply suffering from lack of oil. This condition must not be left unchecked.
If you own an MGB and have never checked the gearbox oil level, now's the time to start. First off, let's bear in mind the simple fact of personalities. MGBs (in fact most British sports cars) require that you occasionally get down on your knees and grovel before them. Think of it as idol worship. Bearing this in mind, wander over to the right side of the car (RHD cars), open the door, get down on your knees, and crawl head first into the driver's footwell.
Once in position, knees on the ground, head where your feet belong, look for someplace really inaccessible. Let's try under the carpet, between the firewall and console on the transmission tunnel. Peel back the carpet to expose a rubber plug (or empty hole as the case might be.) Lurking somewhere down there, half covered by old grease and dirt, is a combination filler plug and dipstick. Pull this out, wipe it off, then attempt to restore it to its original location in the gearbox. Once done, pull it back out and get a proper reading of the oil level. You may find the hole in the trans tunnel is conveniently designed to be a little too small for you to get your hand down in there. Use the stiff upper lip method ... perseverance. Just don't get your finger stuck in the dipstick hole!
If the level is at the top mark, all is well. If not, the best way to add oil is by using some cheap plastic tubing from the hardware store and a funnel. Feed the tubing into the tiny little hole in the transmission, then slowly pour oil through the funnel. Remember, the gearbox doesn't hold all that much oil, so check the level often. You don't want to overfill it. Put everything back together, crawl out, and know in your heart of hearts that the MG gods have been satisfied.
There have been a few stories about cars that worked well until the day the gearbox ran dry and locked up at 55 mph. Make a gearbox oil level check part of your routine maintenance schedule and you'll never have trouble again. Well, at least not until something else goes wrong.
(Thanks for Garth Bagnall for his review of this article)