Over the last 20 years or so, I have rebuilt hundreds of British gearboxes and Laycock overdrive units. Some have been for customers that install them in cars undergoing restoration/repair in their own garages, but most have been in shop rebuilds that I have had the opportunity to watch, sometimes for many years, after the rebuild procedure. I am amazed at the amount of misinformation on the Intergoogles that have confused the car owner as to the correct oil to utilize in their gearboxes, as well as the differences in recommendations of the various car manufacturers during the production span of Laycock overdrives. Triumph and Jaguar for instance, recommended 90 weight hypoid in all their applications. MG and Austin Healey on the other hand, prescribed motor oil in the gearbox/overdrive units, the same as the engine oil. Volvo utilized ATF (auto trans fluid), in their cars that were fitted with Laycock “D” and “J” type units. Speaking of Intergoogle misinformation, I have read on-line testimony from apparent credible sources, that the use of 90 weight hypoid oil in Laycock “A” type units will result in rapid build-up of internal operating pressure that will cause the explosion (implosion?) of the overdrive unit, and that 30 weight non-detergent is the only answer. Total nonsense. Your pressure gauge is busted. If this was the case, one must ask why the Triumph and Jaguar factories, both fairly competent in the production of automobiles, utilized nothing but 90 weight hypoid in all their applications, overdrive or not, from post-WW2 to 1976 without a single service bulletin warning of excessive pressure build-up caused by the use of 90 weight hypoid. I am of the opinion, as a result of many years watching the running in of in-house rebuilds, that most any lubricant, be it motor oil, hypoid or ATF, is suitable for any vintage British gearbox and/or Laycock overdrive unit, provided it is clean and changed regularly. The exception is the use of hypoid oils carrying the GL5 designation, which contain additives corrosive to brass over the long term, for limited slip differentials. Your gearbox probably has brass syncro rings and bushings, although you will not live long enough to suffer the maladies of GL-5 useage. The vast majority of gearbox/overdrive failures are owner-induced, usually caused by ignoring maintenance procedures and changing of fluids on a regular basis. A good percentage of the overdrives I disassemble in the normal course of rebuilding have an alarming amount of sludge build-up in the unit, particularly in the accumulator bore. For in-house rebuilds, I use 85w90 GL-4 hypoid exclusively, in all applications, and have never had an issue. Whatever fluid you choose to use, make sure it is changed at factory recommended intervals, at least seasonally, the adjustments tended to and filters cleaned, and you should have no running issues. As the late John Muir, mechanic, philosopher, author and VW guru stated, “Be kind to your ass, for it bears you”.