oil?

[Mr 5 0]

Synthetic oil is superior to conventional oil but at 84k I would stay with what you have now (conventional) and simply change the oil and filter every 2000 miles, without fail.
You'll get good (not perfect) lubrication, run cooler and pay a whole lot less over time unless you put huge amounts of miles on your Mustang (most of us don't).

For what it's worth: I've used Mobil 10-30 and a MotorCraft filter in my '90 LX since it was brand new. I've changed both at 2000 mile intervals and now, at 110,000 miles (and lots of races) I have no oil burning, no leaks and my head surfaces/lifters are as clean as those on a new car. Works for me.