258 degrees is not all that hot, from my limited experience on the 110. I've seen mine at 268, running 20W50 Redline Synthetic. This is reading it on the LCD dipstick on a 90 degree day, two up, loaded.
I've read the same things about straight 50W Dino oil, but I'm sticking with the 20W50 Synthetic. I just don't trust the Dino oil to hold up as well to the hot temps, and it breaks down faster, particularly when it gets really hot. That's a fact. Whether it gets hot enough to break down is a matter for debate, but I prefer the insurance of Synthetic in the air cooled motor.