Motoring the engine to cool it will do SFA wrt to coking and only promotes rim cracks as per the quoted CSL.
Change to a better oil - Mobil Jet II is marginal at best - works in some engines but not all - try 254.
Is your scavenge strut clean? Have a really good look - it is not round in shape but has 2 flat sides and 2 radius sides.
Low scavenge flow does not help. How many hours has your gearbox and pumps done? There is a procedure to test the scavenge performance from memory.
Flooding of the space will cause build up on the lab seals. I think there is a check valve in the pressure side that works to shut off the oil as the pressure drops below a certain value on shutdown. This check valve can leak. May be wrong as it is about 30 years since I worked on a 250 but hey................
Scav filter will clean the lumps out and is worth every cent.
It's a classic old engine - pay attention to the details though and they work fine.
Last edited by RVDT; 5th August 2019 at 16:53.