i think M10 is the shaft thickness in MM, which would tie in with the washer being slightly larger. So if you purchased the M10 washer, and it was too small, then it apears you have an M18 seal-less drain bolt.
the M is just the standard used in techy drawings to tell ya a hole has been threaded. M10-6H say, would tell ya internal thread of hole diamater 10mm threaded to the tolerance of 6H. the value beside the M being the nominal diameter of the area about to be threaded. not what u wanted to hear but could be useful
then as oddball says lol.
id be tempted to replace washer all the time, and at me learning something from that drawing module at uni.
The early TXB were easier to fill up as they had a level plug where the later ones have the temp sensor. I would look at removing the temp sensor and refilling box from there. Much quicker than filling through the breather.
Try Teeside Autotech Yunis as they can do it mail order - 01642 617626
I used Mobilube 1 in mine but I have got Vauxhall to order the GM stuff this time.
2 different things can be used as level plug.
1. Temp sensor - fiddly as the wire attempts to re-tighten the plug due to the twisting of the wire.
I have always used this method and it takes a knack of loosening with spanner then just spinning the wire between your fingers till it comes out. The thread is fecking long anf ine so its sore arms by the end of it altho it may be easier with the engine out.
2. Mounting bolt adjacent to temp sensor.
Not sure what I am going to use this time tho. "Mobilube 1 SHC 75W/90" or GM. Doesnt matter, both ideal for the job.