Apparently Wilf Green was responsible for the incorrect "translation" from the DDR spec oil - the DDR manuals say to use SAE 30 engine oil if their spec straight light gear oil was not available. I am currently using some EP75 Competition two stroke gear oil in mine (as I got it at a good price) - gives a nice light gearchange & no clutch drag. I have used 10-40 engine oil in the past without problems.
Once spent ages with a late friend's TS150 trying to stop the clutch dragging & dire gearchange back when the bikes were new. Clutch dismantled & cleaned, checked for correct dimensions etc. to no avail. Changed to 10-40 engine oil from the EP90 the dealer had installed at the first service & all was good again. Problem caused by the gear oil change from the DDR initial fill to the Wilf Green recommended EP90 at the first service.
The DDR spec comes out much like ISO46 hydraulic oil, which I intend to try at some point. ATF would also probably work well too - standard on most of the two stroke trials bikes I used to throw at parts of West Yorkshire many years ago.