As this is a '96, there is no sight glass in the receiver/dryer. It's located behind the left headlamp, in front of the washer fluid reservoir.

I'm not sure that 13 is too low for the high-side pressure. Mine runs about that at 100F out when idling. But if it were mine (and I've had this problem as well), I'd add a few ounces and see what happens.

If the refridgerant charge is correct, you may need a new receiver/dryer. Some of the older ones get restricted, causing this problem (according to WIS).