190f is about 87degc which does not sound to hot,but the viscosity of the oil will drop with temperature. If you look at the Castrol classic technical specs for 20w50 the viscosity does drop with temperature,so logically an oil cooler will bring it back up.
Viscosity, Kinematic
40C
ASTM D445
mm²/s
157.71
Viscosity, Kinematic
100C
ASTM D445
mm²/s
17.3
The issue in adjusting to compensate for when hot ,will be when cold you may have excessive oil pressure which is not good either/you have to take care till warmed up.
I would adjust to suit the type of driving you do the most,but in the back of my mind it should likley bet set for "operating conditions,and not cold settings as per Bristol procedures,but then someone with correct advice will be along later.
One other thought is that you are running the standard oil pump rotors,and could/should upgrade to the MGB rotors. There was an article (i could email) ,and I did this to my Bristol 400 oil pump (Using a 100d2 pump body ) and have much improved oil pressure now.
Re the backfire and no power below 2000rpm (and this is based on Bristol 400 85c engine experience) --I would check the distributor timing advance base plate is functioning & dist body earthed(400 has sliding manual advance/retard as well),and as you have put new carbs on,the float heights--you need to make a tool to fit in the carb body to get all fuel levels correct,as the engine is sloping front to back as it sounds like the carbs are not coming of the idle circuit onto the main circuit smoothly.Carb balancing is also worth checking. I also made new gaskets where the carb body meets the head and used hylomar to stop any leaks there that might weaken the mixture.
Graham