I've had the urethane oil sight tube in place for 12 years. It is still neither hard nor soft, only some yellowing. For oil or fuel it holds up well. I just replaced the piece used as a sight tube on the 15 gallon wall mounted fuel tank...it was still fine, but had yellowed to the point where it was getting hard to read the fuel level. This despite being used with B99 fuel for several years.
It's a different situation for tubing exposed to sunlight; my engine room is enclosed.