I do use the diff casing to get the car lifted into the air using trolley jack (spread the load using a square piece of laminated board), but as quick as possible get axle stands (again with wood off cuts of appropriate size) under the sill jacking points… and lower slowly until most of weight on axle stands. Leave trolley jack under diff as belt&braces, plus wheels under each sill when removed… you can’t be too careful!
I’ve never used the vertical reinforced sill section to bear the weight of the car though; just inboard of the vertical reinforced sill is a kind of ridge and furrow area, maybe 4”x2”, that I use to spread the load over wood and axle stand. Have I been doing it wrong for all these years??? I do always have a good listen as I drop the car onto the axle stands though… crunching rot wouldn’t be welcome!
But when you lift the car anyway the rear wheels are hanging. So I don’t think that would chew the seals up. More like I read, it could fracture the diff mountings.
I’ve stopped this method where possible and jack up on the sill jacking points but I did jack on the diff when confined to the garage recently, couldn’t get to both sides properly with the trolley jack to feel safe.
The diff’s almost always one of the best places to jack a rwd car, it’s built to take the stress and the bushes etc aren’t directional- if they’re strong enough one way, they’re strong enough the other. it’s just a bit awkward on most mx5s.
Bushings fail over time of course, especially oe rubber ones, so a picture of a failed bushing doesn’t mean a thing
