Unlike the engine, the transmission does not have a pressurized oil system with a filter so the fluid specified is non-detergent. One of the goals of detergents in oils is to keep debris suspended so that it can be filtered out. For non-detergent oils, you want the debris to fall out of suspension and stick to the bottom or walls or a magnet so that the debris doesn't circulate around ruining stuff. So often times you see a lot of nasty looking parts while the oil itself doesn't look so bad. I work on axles a lot and it is the same scenario.
Also, you could use RTV on that joint with a high degree of success. Cleanliness is important but it appears from Steve's pictures that the parts are designed to be compatible with a liquid gasket (see the chamfer?). EP additives are very surface-active and are aggressive with the RTV so make sure you use the correct oil. One advantage of the RTV joint is that there is metal-to-metal contact so the diff bearing preload will be very stable. With a paper gasket, if it creeps .001" you loose a thou of preload. Looks like Steve has a fresh tube of Ultra Black which I have used with good success (albeit in other applications).
Agreed on all points.
The transmission pictured had a couple of problems; first, it had the wrong gear oil. Which promoted the second problem. It was run very hard (in a track car) and over heated the gear oil caused excessive wear throughout the transmission. Nor was the gear oil changed regularly so it suffered from both break-down and excessive contamination from the wear. The oil came out black and heavy with contamination. The bearings, synchros, sliders, forks, gears and gear sleeves were all junk. All worn beyond serviceable limits.
The X transaxle does not have a hypoid gear so there is no need for an EP gear oil. I can almost always tell when one of these transmissions has had EP in it because of the resulting leaks.
The X case was designed for use with a gasket but work quite well with RTV only. The OE gaskets are quite thin so the diff preload is pretty sable. When I build racing transmission I never use gaskets, RTV only. The window for diff preload on a racing transmission is only a couple of thousandths. Outside of that window the bearings _will_ fail prematurely from the heat expansion. Using a gasket makes this problem worse. RTV only make the case interface more stable and reduces the change in tolerances.
I built most of the current batch of units (all for street cars) with gaskets as the shim stacks are sized for that clearance so its much easier to set them up with the standard range of shims. Also, I had a couple of people ask about the lack of gaskets. It looks more OEM with the gaskets in place. I do use RTV with the gaskets. Close examination will show a tiny bead of black RTV where the case meets the gasket.
Another word on oils. I know the factory service manual says to use GL1. I haven't used GL1 in probably 30 years, nor should you use it for any reason. As a gear oil its at least 3 generations out of date. There are several modern oils that work far better. I always recommend Redline MTL for street cars and MT90 for track/race cars. MTL/MT90 is a modern synthetic formulated to work with a synchromesh transmission. It works very well with the Porsche Type _and_ blocker ring type synchronizers as well as the various bearings. Its stable over a wide range of temperature (MT90 shifts its temp range up slightly for better high temperature performance) so it works well in your street car. It will flow better on a cold morning and not thin out excessively on those long summer drives. There are others that work reasonably well. GM, Castrol, Pennzoil all have a good synchomesh fluid (I assume that GM's synchromesh is probably one of the other two simply labeled as GM.
EP oils don't work very well with the sychronizers. And please note that there is no brass in the X1/9 transmissions. Nor do any of the "90 wt" or multi-viscosity (75/90, 80/140, etc..) gear oils. Those are primarily intended for older transmissions or differentials.