The OP said they are using both preheater and the warm oil. I think it can't hurt.
Years ago my '62 C was parked on the ramp in MA for a couple days when the temperatures were hitting highs of -10 F.
There was no heated hanger space available. The morning I wanted to leave I had the FBO try to preheat the engine, but their preheater ran on propane which would not vaporize to keep the burner running for more than a couple minutes at a time. We plugged the sump heater in and I did exactly what the OP does; heated a couple quarts of oil in boiling water. The '62 C had a very specific cold weather starting procedure in the owners manual which included pumping the throttle and flipping the prop. Between the heated oil, sump heater, and many attempts to keep the FBO's preheater running we got the engine warm enough to feel warmth on the cowl and register on the oil temp gauge. Despite the predictions from the small crowd watching out the window, it started right up and got immediate normal oil pressure. It was a nice smooth flight home.
Yes, the bush pilots will drain the oil and take it inside to warm up (or used to anyway) but they warm ALL the oil. If you leave frozen oil in the sump it can clog the pump. With multi viscosity oils and 'normal' cold temperatures (not -40 like in Alaska) that should not happen. Personally, I think adding some warm oil can't hurt and may help, but I would still flip the prop to help get it mixed up a bit.
I solved most of my cold weather starting issues by moving to Arizona. I highly recommend it.
Jim