You need to be a bit careful with a viscous LSD - Redline 75W90NS is recommended for LSD's but is a bit technical and you're supposed to use friction modifiers and work out the amount by trial and error - sounds a bit too much like hard work. It's OK without friction modifiers for normal diffs. Redline 75W90 is stated to be too slippery for Synchromesh gearboxes and is specifically for diffs - but viscous LSD? Dunno - I have a normal diff. and I use 75w90NS (I wanted 75W90 but none in stock at the time)

I shouldn't have mentioned MT90 for diffs - it is a GL-4 spec whereas the specified oil for the diff is GL-5

You could ring Eldon House Automotive (Kent) 01732 866885 - I think they're the Redline distributors for UK.

Or AMS have a rather simpler product: 75W-90 Synthetic gear oil (auto spec GL-2 through GL-5) but I'm not certain that it's recommended for diffs.

Change the diff oil - if it hasn't been done for some time you'll be surprised at the treacle that comes out!

BTW - D-i-Yers you'll need a 23mm socket for the filler plug (? one of them anyway - the other plug is 22mm - both take the same sealing ring - as does the gearbox drain and the sump plug) 23mm sockets are nearly as rare as rocking horse droppings, but they are about! (1/00)
The V-LSD has NO special requirements in terms of oil, because the slip limiting part it is a sealed unit. I would recommend a good synthetic diff oil (I use Castrol). (1/00)
When I replaced my open diff. with a clutchtype LSD from Mazdaspeed, I have done some testing (was forced to) on diff. oil. 75W90NS only is a definite NONO with such an LSD. Friction modifier is needed. You can fine tune that yourself, but it's a lot of time to get thing right.... I now use Valvoline diff oil for rear axle driven with LSD. Works like a charm. At least no diff chatter. For the V-LSD frictin modefier is not needed. It's a sealed unit. (1/00)
I suggest you look for one from a -94 1.8 on car. These are bigger. Don't if the 94-on 1.6 uses the bigger diff also. Anyone? Just to be safe, I replaced the little diff on my '91 with the big diff BEFORE it broke... (1/00)
Question - how can you tell if you have a limited slip differential? Reading "Renaissance Sportscar" the production history section seems to say that when the 1.8 roadster was introduced in Japan (1993?) all five speed models were fitted with Torsen LSD's! Is this likely or have I misread? miata.net suggests that all casings (limited slip or not) were the same with no identifying marks (mine has cooling fins but they say this is a red herring). I ask because when looking at diff. oil the Castrol EPX 80W/90 I found says that it's good for all differentials "excluding limited slip". (3/00)
This exclusion is only valid for clutch type LSDs, which, AFAIK, were never fitted by Mazda as a standard item. The Torsen and Viscous LSDs function is not impaired by oils which are not recommended for clutch type LSDs. (3/00)