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A question of lubrication

I have been, over the last few weeks, working on a rather rare de Normanville "Safety Gearbox" fitted to a 1930s Humber Snipe "Pullman". Needless to say it is the first one I have seen, if you are not familiar this is an epicyclic gearbox, all gears contained on a central rotor with engagement controlled by pressurised oil through a piston block, brake shoes and control rods. It came in with no second gear, the fault traced to a newly manufactured part being fabricated incorrectly and breaking, this has been repaired and the whole thing reassembled and back in the car. Now to the lubrication question, the workshop manual calls for "Castrol H" and states that 'thin' engine oil can be used temporarily but should be immediately drained out when the correct lubricant is available. Other than a gear-set that is much the same as the Laycock-de Normanville overdrive the only other related factor is, I think, that the seals in the piston block are leather. I have been unable to find any information online about "Castrol H" so what I am asking is, what would you use?

The photo shows the gearbox from below with the sump, oil pump & filter removed, the piston block runs down the left side, the 4th speed piston down the right and you can make out the rotor, brake shoes and control rods running down the centre (with the pump having been mounted on the long studs at the top).



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