I drive my 380 in "D". RPMS seem fine and not that much "hunting" of gears when in 4th gear or whatever it is. I have not even noticed, does the 380 have overdrive (or 4th?). It runs at about 2200 rpms at 60-65 mph.

My 69' Caprice and 73' Grand Ville have 3-speed automatics and they have torquey engines, so leaving them in "D" is not an issue. I can go about 65 in either and barely be at 2000 rpms, but power is still fine. My Suburban was horrible however. When in "OD", it would only be at about 1700-1800 rmps - even at 65-70 MPH! When on the freeway, it did a lot of "hunting" between 3rd and overdrive. My 89' Pontiac Safari wagon is the same way - LOW rpms even at high speeds. Also low ion power, so there were times I drove it in "D", rather than "OD". I do not even drive it anymore because of the LOW power.

Starting new thread about that too.