Oil type and temp Questions (houston area)

[AggieDriver]

I bought the 09 370z yesterday. This is my first "real" car after driving handed down 4x4 suvs and farm trucks.

I understand the oil temp issue, knew going in. I'm a recreational driver, not planning on taking the car to a track for anything serious or at all. Just enjoying the car on the streets. I'm seeing oil temps from 220-240 just driving around town and highway with ambient 60-70. Oil cooler is the top of the list, right up there with a radar detector to hopefully keep from paying the christmas bonuses for the troopers.

Questions- as a decidedly NOT diy kinda gal, anyone have a suggestions for where I could get an oil cooler installed in the Houston/College Station area? I'm leaning toward the smaller Stillen. Also- after seeing single digit days last winter, should I be putting in a sandwich plate even though this is a warm climate most of the year? Would that answer change if I were to move to a more temperate or cold climate?

Oil type-- Car was purchased used (25k miles). At that point is it a poor choice to change oil type? If not, what oil should I be running in it?

Also--- apologies if my clueless blonde shows through here, when getting an oil change how mindful should I be post-cooler install about a dealer or shop putting the correct amount of oil in it? Do I need to be more mindful of where I have my oil done? Good mechanic vs dealer vs jiffy lube and the like?

Apologies if this has been answered before, wasn't at all successful in searching.

Thanks in advance!