the fade effect is only visible when lum < 50 but I think it must be my eyes
Or that fact that you aren't using a current-limiting resistor. Or, are you?
so I can't use other PWM than 5 with IRemote ?
There are 6 PWM pins, using three timers. Using the IR library should still leave you 4 PWM pins. I thought that 3 and 5 shared the same timer, so it's strange that 5 worked, but 3 didn't. 9 and 10 share a timer, as do 11 and 12. Or, at least I think that's the way they are grouped. Try all 6 pins, and see for yourself.