I checked my '85 owner's manual, and found something odd.. it lists two engines for everything (like it should), but under plugs it shows 3.8(229), not 4.3(262) like it does for air, oil, pcv, and pcv filter. It does say R43TS for the V6.. But to answer your question, maybe the R43TS are a little 'cold' for some driving habits and they've since revised it to a 'hotter' less foul prone heat range. Unless you race it or do a lot of high speed interstate driving, I think you'd be fine with the 45's. Or 43's. Anybody know what number is listed in an '88 owner's manual? Ha ha ha.
Sorry-- I just reread your post and saw the 'C' in R43CTS. The C stood for commercial or something like that, they were more resistant to fouling. They are obsolete now; maybe that's why they step up a heat range in cataloging now.