Use at least three tanks of premium gas. It has a cleaner in it. Also, if it's an automatic tranny, keep it in 2nd gear, rev it up to the same revs you would as if it were in high range going 90 kph.

If it's blue smoke, change the oil. I'm no fan of additives, just top quality oil will do.

Never rev an engine in park.

My suggestion would be, get her a hybrid and you keep the 300. Driving a 300 to and from the doctor is like dusting crops with a B-29