Welcome to the site! Your truck would have had R-12 refrigerant in it when it was built. Manufacturing of R-12 was outlawed in the mid '90's & R-134a became the refrigerant used in the new vehicles from that point.
This leaves you with a couple problems that you will have to work through. 1) The cans you see in the local stores is R-134a. 2) Since you just bought the truck, you may not be able to tell FOR SURE what is in it now...many systems of that age have been converted to the 134 which could be good since it is available, however LOTS of "funny" stuff went on in the late '90's-early 2000's with other refrigerants being available...some were even highly flammable!
Look for Red & Blue caps with an Aluminum adapter on the charge ports & a Retrofit Decal on the rad.support or hood...these were to be used when the conversion was done, however I'm sure they didn't all get them
The oil for the R-12 & R-134a are different & shouldn't be mixed. I'm assuming by now you're getting the felling that this is a bum deal & no where near as simple as getting a little can at the local store.
Shops at the time often kept a spare, "junk only" tank to put all of the refrigerant/oil that was reclaimed from a vehicle. I bet there are many collecting dust in the back corner of a stock room.
The best you can hope for is that it has been converted & properly labeled. It would be best to take your truck to a licensed A/C repairman & ask for help. My Two Cents, Lorne
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