The perceived wisdom is to run these old style engines with at least 28% oil. Read Randy's explanation of fuels to use, which is pinned above. Running with some synthetic is also a good idea. Stops the castor crud hardening and doing nasty things like freezing the rod to the wrist pin.
I have a brand new old stock Veco 19 complete with the car conversion bits. When I get around to running this in, I will be using the 28% oil with maybe half and half synthetic / castor. brew.
If you can't hot restart the older engines, they probably are pretty worn. One trick is to squirt some fuel over the head to cool them down quicker. Used to do this when I was a kid and had to put up with old worn engines!

Regards,

Andrew.

After two pints of 30% oil fuel , I tried the 23% in the Twin . Wet Rag useless .
back to 30 and it howled .

They seem to take a bit of time to loosen off , so don't hurry it .

Got 88 mph timed in the speed thing , on 40% nitro the other day.
8x6 props 54 inch profile stunt thing . . ( .40 speed class . 52% !  )

10 , 15 or 20 % nitro when run in . Not as consistant or easy to set on 5% .