Who decided that this could be ignored? I don't know how many 30 year pilots I've flown with have decided that oil temp must be in the green before run-ups or take off.
Agree - I have seen this many times. I recall getting a dual check in a Cessna 152 at a one man band flying school.
I was fairly experienced as a grade one instructor at the time. My instructor, who was also the flying school owner and an LAME, insisted the oil temp be in the green before run up. He wasn't interested in what the manufacture POH said about in colder than normal OAT the oil temp may not register a rise until after take off. There was more to come.
After I leveled at 1000 feet after take off, he insisted on a wide circuit and long downwind leg to "allow the engine to cool after the climb at full throttle". Then he really arced up when I reduced power to idle on base for a practice glide approach as he said that would cause shock cooling. He wanted nothing less than 1500 RPM on base and final "to keep the engine warm" OAT was 20C... No closed throttle approaches permitted he said in
his Cessna 152.