I don't think Mugen will leave us hanging if the diffs turn out to be a shade fragile, alloy or steel gears may already be in the works surely?.

With gear oil, I know the thinner you go the more load the gears see (shock load as well the way a nitro delivers power through a clutch), so I'd be reluctant to go thinner than kit in the back end of the 5, but some of that has to be how you drive it and ho much power your using.