Interesting observations, @dlearl476. FAIK the Raby DTM has no thermostat nor provisions for same. In my case I've not yet been able to run the engine over the road, so I can't say yet if it behaves similarly to yours—but it's a strong possibility. The gauge and (original) sender that came with it were supposedly matched and calibrated by the good folks at North Hollywood Speedometer, and I have no cause to doubt their work.
As for oil: Brad Penn "Green" 20-50 is specified (very adamantly, I might add) by the engine's builder. I see no reason to vary that, given his dire warnings.
So the plan right now is to get the car registered and legal & see how she does underway. I'll revisit the issue then, though I do plan to invest in an infrared thermometer in the meantime to spot-check head temps and such.
Yeah, I get that BP thing. Brad's devotees are, shall we say, religious in their fervor.
FWIW, I'm the same way about Motul, but the straight weight Motul Classic is $12/L vs $8.95/gal for the Rotella. A real shame, as I can get the 300V for free. But like I said, the motor sounds awful with it.
How about the oil thermostat? If you don't have that, cold oil will circulate through the cooler and further impede warming up.