There was no 75W90/140 gear oil in the shop, but there was ATF… which I used to give the SG-8R31 a bath.
And… it makes zero difference: In either speed 4/5, after giving one revolution of the crank by hand, the wheel still stops spinning after about 5-6 seconds.
If I get the opportunity, I'll ask a Brompton rider if an
SG-C60xx (00, 01, 10, 11) offers less friction and is worth the expense + rebuilding a wheel… or it's just what happens when you stick it into a 16" wheel.