Well spotted GB - it's called politics
Last summer I put 8 fl oz of Halfords ATF in mine and it was ok without any noticeable problems . . . .
Then I changed it in November (just for the winter months) for Castrol GTX High Mileage SAE 15-40 because I had a full 5L of it . . . .
Then I changed it again in February for Morris Golden Film 20W/50 after changing from original 6-spring clutch to SRM 4-spring . . . .
Then again in May for Morris Suspension 20 WT because I read somewhere that it's ideal for the primary chaincase because of its anti-foaming poperties . . . .
Then at the beginning of June I noticed a slight
weep from the drain plug (on my non-leaking-oil GF) so fitted new washers and then went back to Morris Golden Film 20W/50 again . . . .
I've not noticed any difference in any of them on the performance of the clutch
.
(Edit): As Roy has said with the SRM clutch kit the instruction specifically advises against ATF.