Thermostat and Gunk from Exhuast

[IdealConcepts]

I posted this on another site but wanted to get opinions from Type R owners as well.

Ok, so I have a 2000 Type R, with about 72K miles. For the last 2-2.5 years or so it sits in the garage and I drive it about 4-5 times a month at least 20 miles during the month. Sometimes I drive more often and for more miles. It was a daily driver from about 2002 to about 2005 I have been using Mobile 1 Synthetic for the last 2 years, and change the oil ever 3-4 months at the dealer and they top off all the fluids and last year I had them do a 30K to keep the car up, and I had the 60K mile done about 18 months ago. Only mods to the car are suspension, HID lights, stereo system, had a AEM cold air intake but removed that about 2.5-3 years ago and put the stock one in.

I have two issues. My 1st issue is the temp gauge in the car seems to do something odd before it reaching normal operating temperature, which takes about 10-15 mins. The gauge basically seems likes it goes up close to normal operating temp after about 4-5 mins but them goes back down if I get on the freeway or hit a stretch of road that I am assuming is cooling the car back down or could this be related to using synthetic oil (I am under the impression synthetic oil lubricates the engine better so it could take the engine reach normal operating temp and could be causing the odd thing with the temp gauge)? I didn’t pay much attention to this when I first switched the car over to synthetic oil, also the thermostat is the original if that matters. This odd thing with the temp gauge goes away after about 15 mins or driving, which I don’t know if is related to my second issue?
My 2nd issue is there is black junk coming from the exhaust and the readings on my last smog test for HC at 15mph was very high, 1 more point and it wouldn’t have passed. Here are the emission readings from the smog test back in November of 2006. (oil change was done in Mid October 2006).

_______CO2% 02%
_______Meas Meas
15mph 15.00 0.00
20mph 15.00 0.01

______HC HC HC______CO CO CO_____NO NO NO
______Max Ave Meas Max Ave Meas Max Ave Meas
15mph__60__9__59__.53__.02__.14_459_57__26
20mph__43__7__28__.51__.03__.05_746_50__13

I haven’t had the spark plugs replace, unless that is done at 60K at the dealer, and haven’t had the 90K done yet. All the readings look good except the HC measure at 15mph and the black junk coming out of my exhaust. The person at the smog shop said it might be the oxygen sensor but the check engine light has never come on. So before I take it to the dealer, what do you guys think it is? Is it the oxygen sensor, catalytic converter, exhaust, spark plugs?

So what I want I need your help with is 1. is this normal with what is happening to my temp gauge and 2. what should I do to stop whatever is causing the black junk that is coming out of my exhaust and reduce the HC readings at 15mph. Anybody with suggestions, comments, thoughts, recommendations, ideas, stories, experience with this or something similar? Thanks in advance.