In school, I was taught to design for 180°F max operating temperature. In practice, it is not unusual to hit 200°F - 210°F. Much over this for prolonged periods, and you could run into oil breakdown issues.
I am assuming that the 90-100° over ambient that you are hitting is in °F, correct? If so, and you do not live in the desert (why would you need a brush cutter?? LOL), you should be okay.
HTH - SR