My Haynes manual just says to "never use more than 2 cans", which
amounts to 24 oz. My system was so low that the clutch would not
engage until I added 1 can. I added a second 12 oz can to get the
low-side pressure to read about 30 degrees on my gauge. I added oil
last year, so I didn't add any this time.

I was under the impression that the head pressure goes up when there
is too little refrigerant. Maybe I got that wrong.

I'll look for a sticker under the hood. Thanks!
>
> "A few cans" may exceed system capacity. Since you don't say what "a
> few" is...look at the weight of the R-134a in the can, multiply times
> how many cans. Now, how many total ounces of R-134a and oil are you
> putting in? Compare this to the total system capacity, which is on a
> sticker under the hood. If you have too much, the SCREECHING may be the
> compressor clutch slipping.