The answer is "depends"
What kind of cars are you working on? For cars with harder clear coats, I found optimum impossible to use with PC. I know that optimum has worked for a lot of people with PC though, but it just won't do the trick for some.
As far as detailing in the sun goes, this kind of depends on where you're located. Optimum is known for its very long work times, so its easy to use in the sun compared to
Menzerna polishes. But if it isn't that hot out, I've had no problems at all polishing with SIP and 106FF in the sun.
My advice is get SIP and 106FF and work in the afternoon or morning if you have to. These polishes are a major step ahead of optimum for use with a PC. They cut faster AND finish better, what more could you want.