Spot on Bodi- the gearbox is on a separate oil "circuit". Furthermore, on the 400 it is possible to go through the whole gearbox without touching the top end by just inverting the engine & removing the bottom case. I had a look at my stocks but don't have any easily accessible bare cases to see how the oilways run, but anyway I think that going through the gearbox is the only sensible option here, as it will enable checking for any blocked gearbox oilways and in any event will allow a new plug/seal to be fitted. Shortcutting this approach would be a bit risky imho - as well as the obvious risks of mechanical damage from a lubrication failure, there's the safety aspect of a potential gearbox seizure to be considered.
