Hi,
I don't think an official figure has ever been given for the oil pressure on postwar Douglases, but from experience, 10psi seems to be about the normal on a warmed up engine. Volume seems to be more important than pressure! With the Dragonflies, even with the larger output pump, the lack of volume seems to be the reason most of the oil escapes from a worn front bigend under the centrifugal force created at high revs, leaving the rear bigend starved. The only plain bearing in the Douglas motor that is pressure fed is the front main bearing, for which Mr Douglas recommended 3 thou clearance (presumably to allow the oil to flow through it, and permit some flexing of the crank). The rest of the bearings are splash fed, so should also have enough clearance to allow the oil to get in and do it's job - even a slightly tight camshaft bush can result in a motor being less willing!
With regard to the revs these engines will run to: The low compression Comp engines had a 5,000 rpm ceiling, Mark engines went to 6,000rpm, and the Plus engines were good for 7,500rpm. Later works Plus motors were further tuned and were reputed to be capable of 11,000rpm - but nobody seems to know for how long!!!
Regards,
Eddie.