Classic gearboxes and rear axles use very different oil specifications from modern transmissions, and using the wrong product is one of the most common ways to damage an otherwise sound classic. The synchromesh, gear materials, bearing types, and seal materials in a classic transmission were designed around a specific era of lubricant chemistry, and modern synthetic gear oils will not produce the right results, particularly an EP gear oil used in a gearbox that requires a non-EP lubricant, which attacks the yellow-metal components. The gearbox, overdrive, rear axle, and automatic transmission each have specific requirements reflecting the metallurgy and tolerances of each assembly.
Gearbox & Overdrive Oil
The single most important point in classic-MG transmission lubrication is that the A-Series and B-Series gearboxes fitted to the Midget, the MGB, the MGA, and contemporaneous classics specify a 20W/50 engine-grade oil rather than a modern hypoid gear oil. The reason is that the synchromesh cones in these gearboxes are made from yellow metals, bronze and brass alloys, which are attacked by the sulphur and phosphorus extreme-pressure additives in modern hypoid gear oils, so using an EP gear oil in an A-Series or B-Series gearbox progressively erodes the synchromesh cones, leaving a box that crunches on every shift and eventually refuses to engage. The correct oil is the same Castrol Classic 20W/50 multigrade used in the engine, a low-detergent mineral oil, so an owner topping up the gearbox uses the same product as for the engine. The overdrive unit shares its oil supply with the gearbox on cars fitted with overdrive, so the same specification applies across both, and on the MGC and MGB GT V8 the overdrive's internal hydraulic system relies on this oil for pressure generation, modern friction-modified oils potentially interfering with overdrive cone-clutch engagement and causing slow or intermittent operation.
The MGC manual gearbox may alternatively take a straight EP80 or the multigrade Hypoy B 80W/90 where the handbook specifies, the multigrade offering improved cold-start fluidity, so the original handbook recommendation is the authoritative reference for each car.
Rear Axle Oil
The MGB, MGC, and contemporaneous live-axle rear ends use a Salisbury or Banjo-type axle with a hypoid crown wheel and pinion, and these do require an EP-rated hypoid gear oil, typically a GL-4 grade EP90 mineral for the standard axle or a heavier GL-5 specification for axles that have seen significant load. The EP additives in the rear axle oil are sealed away from any yellow-metal components, as the axle internals and bearings are steel, so the EP additives protect the hypoid gear mesh without attacking anything else, making the hypoid axle the opposite case to the gearbox, the EP additive being essential rather than harmful. Using a non-EP gear oil in a hypoid axle is the wrong choice in the opposite direction, as without the EP additive the high-pressure sliding contact between the crown wheel and pinion, where the pinion axis is offset below the crown-wheel centreline, will score the gears under load. The rear axle oil level is checked at the filler plug on the differential housing with the level at the bottom of the plug hole, and dark, metallic-contaminated oil drained from the axle indicates gear wear and should prompt a closer inspection.
Cars fitted with a Quaife or other limited-slip differential require a friction-modified fluid such as Transmax Limited Slip 85W-90, as standard EP gear oil without the correct friction-modifier additives is not suitable for limited-slip units.
Automatic Transmission Fluid, Steering & Chassis Lubrication
Automatic gearbox cars fitted with the Borg-Warner Type 35 require an automatic transmission fluid rather than a gear oil, the fluid serving as both lubricant and the working medium for the torque converter and the hydraulic control circuits, with Castrol Classic TQF being the specified fluid, and the wrong fluid type potentially causing slipping, harsh changes, or seal damage. Pre-war and immediately post-war steering boxes and chassis components, including those on the MG TD and TF, specify semi-fluid grease or plain mineral oil rather than modern grease, covered by the Castrol Classic semi-fluid range, while for the later cars the front-suspension swivels and trunnions specify a king-pin grease rather than a modern multi-purpose grease. The wider range includes straight ST90 non-EP gear oil, EP140 for heavier rear-axle applications, and lighter manual transmission oils where specified. Gearbox, overdrive, and axle oils should be renewed at intervals appropriate to use, typically every 12,000 miles or annually for cars in regular service, fresh oil making a noticeable difference to gear selection as the synchromesh rings operate more effectively in clean oil, and the oil-finder reference covers the correct product for each car and application.