1976 DT100 has been riding fine, but making a bit of white smoke so I drained the oil to replace the R/H crank seal, the oil looked all OK, but when I took the case off there was this layer of grey silt in the bottom of the case.

What has caused it, any ideas?

If you try a magnet on that “dust”, you’ll prove it to be metal or not.

It is in the oil, so just wets the magnet anyway.

Perhaps then it’s aluminum that’s showing up.
Yer oil doesn’t have some special additives?

Without seeing it, the main two causes for gray trans oil deposits are moisture and clutch material.  

Moisture can be from condensation (and not riding enough to get the oil good and hot) or even washing the bike.  Is your plastic trans vent elbow in good condition with a fairly long vent hose that goes back across the engine and down towards the ground?  

Water is heavier than oil and settles to the bottom of the oil.

Or it's just clutch wear debris.  "Clutch sludge" is fairly common to find.

I'm with MarkT, my first thought was clutch wear debris.

Thanks guys. I'm sure you're right about the clutch material.

New friction plates not that long ago are now down to 3mm.

I think the clutch must be dragging when not engaged.

New clutch friction plates ordered, but what can I do about the dragging. I have the little rubber rings still in between the metal plates, do others keep them in or not?

1976 should have been the first year for 3mm clutch frictions.  Earlier models have 4mm.  Verify your year with the VIN.

Check and make sure you have the correct number of frictions and steels, someone could have left something out in an attempt to use the earlier 4mm frictions?  I've seen that happen more than once.  (Okay twice, which is still more than once! LOL). 

I leave the old rubbers out because they can swell and/or harden and contribute to slipping.   A friend I helped once bought brand new rubbers from Yamaha when we did his clutch and they seemed thinner and definitely softer than his old ones.