My first set of GP's lasted 90K; I'm on the second set and have (oddly enough) 90K on these with no problems. Plugging it in does make my truck start easier on cold days, but leaving something plugged in longer than 3 hours is wasting money.
I've never used fuel additives, seem to have good luck just trusting what's in the fuel. I run Delo 400, and the oil needs to be changed no later than every 5K miles. The shear forces the oil goes through in the injectors is pretty hard on it.
Also, I got a deal on bulk fuel filters and change mine with every oil change.