hmmmm one thing at a time...

your temp gauge i wouldnt worry about, but it sounds like your thermostat is stuck open. you mentioned that your car takes forever to warm up which is a tell tale sign.

synthetic vs standard oil wont really affect "cooling", if at all.

i ran no thermostat in my teg since my engine bay got pretty hot, especially during the summer.

dont go cheapo and get some autoparts store one, just go to the dealer. the one for a 99-00 civic Si will work, should be cheaper than going to an acura dealer.

the black stuff coming out your exhaust can be a number of things...

i would start off by changing your plugs. high HC numbers means that theres unburnt fuel getting passed your cat. if your engine is healthy, then its your cat...if its not the cat, look into whats going on with your air fuel mixture. theres tons of things that contribute to high HC's....

did u smog your car with a fresh oil change?