i think there is no difference between manual and auto engage for the compressor, you might wear out the button though :D but as for the compressor, its either on or of, cuz its just a switch. and plus the clutch should slip a little when it engages, and thats why it wears out. at idle or what ever engine speed the computer lets it engage. so i would just put it on auto and set the temp to a comfortable setting and forget about it.