The mini was a brilliant little car, and no you didn’t need to drop the engine to change the gearbox oil, as it was designed to compete with the motor bike and side car, the best description for the engine/gearbox arrangement was “unit” i.e the engine oil was the gearbox oil. Whilst the oil needed to be changed more regularily than a seperate arrangement, the gearboxes were bullet proof with the exception of the drop gear, which due to being helical had a tendancy to force its way out of the casing when the engines were extensively modified (the cure was straight cut gears). I personally never had one cut out in the rain ,but by virtue of the fact that the distributor was located at the front it needed waterproofing when fording. I did read somewhere that part of the Swifts problem was due to an un-named subcontractor making a tiny modification to the fan blades of the Avons subsequently fitted to the Swifts.