020 5th Gear Damage
It is important to keep track of the amount of oil in the 020 because of the design, the 5th gear set is the first set to run low on oil. When the trans starts kicking itself out of 5th gear on the freeway, it is getting closer to losing 5th gear completely.
The design of the end of the trans means 5th is fed oil by only a few small holes, and low oil in the rest of the trans will begin to starve the 5th gears, and they'll start to get hot.
You can see all the holes here, a big one to the right of the splined shaft (it lines up with the side filler plug), then a smaller hole under the splined shaft...
Two examples of 5th gear, the one on the left has been ran low on oil, the one on the right is OK. The heat causes the gears to turn dark and blue...
A closer look at the driven gear reveals a rough textured surface, which is actually melted plastic. You can also see that the profile of the splines inside the good gear are square and sharp, and the splines inside the bad gear are rounded and worn away.
This is because of the heat generated between the gears. The heat tempers the metal, softens it, and causes the splines to wear.
This pic shows the thrust washer removed from the bottom of the driving gear sync assembly. The plastic on the driven gear was spun out from this gear when it melted. The plastic was once the retaining cage for the gear needle bearing...
The problem gets worse when the needles begin to move around and clump up on one side because the cage has melted away. This allows the gear to tilt on the shaft, and it causes it to mesh with the driven gear at an angle, which causes more heat, tempers the metal, and allows the splines to strip out, making 5th gear useless.
The smaller mass of the driven gear means it will heat up faster, and be the first to strip out. The gear shown in these pics was pretty close to stripping the splines out completely, and just spinning on the shaft.
The spline profile on the output shaft can also be damaged, making a replacement gear fit loose on the shaft, and could cause it to strip in the future from the slop.
When the gear strips out, the steel that was once the splines is distributed through the rest of the trans, meaning the entire box needs opened to remove all of the steel, a flush simply won't get it all.