What brand of fluid did you use, and was it specifically labelled dexron VI or just compatible (multi vehicle)? When I drained my 15 for the first time at 29k miles, I only got a little over 3.5qts as well. I refilled with the same amount, then started car, drove it till hot, pulled check plug (while car was running, level and in park), and added 3/4 quart of valvoline dex VI until it started coming out the check plug. Car still shifts like crap (mostly just hard downshift when slowing) even after 2 drains and 4k miles. I'm going to do it again in another 5k miles at my next oil change.