I used dow33 on the last reg I rebuilt. I'll be using chemplex 710 on the next one though. Someone like 3 years ago did some testing and found that it was the best reg lube (done by testing the refill rate). Cant remember anything else about the test, dow 33 was ok. Synthetic oil was the best but they only did this test on HPR's and not take regs so I would skip the oil for the tank reg.