No oil makes it sound better. I tend to buy a few cans of 134a whenever I see it at a decent price with no additives. I'm sure I'll use it eventually.

But you should still consult the chart, even if you're only checking the low-side pressure. On my car, the low-side gets lower when the system is under a greater load, according to the chart. It would be very easy to overfill under these conditions.

But you're right that outlet temperature is a good way to get a decent guess. Just don't keep going once it gets to 40F or below.