Just finished up a long road trip from San Diego to Portland, lots of I-5 mountain passes in my 99 V70 T5M. I noticed a very disturbing problem. If I coast down a long grade with the car in gear but my foot off the gas to keep my speed in check, at the bottom of the hill, when I hit the gas for the first time in 2 or more minutes, I get a huge plume of white smoke out the tail pipe, which lasts for about 2 seconds, then completely disappears. I can't seem to make this cloud of smoke appear in any other circumstances, either at cold start up, or just hitting the gas hard during normal driving. For some reason, that long coast seems to build up something that then burns off when I reapply power.

I've noticed the car does slowly loose coolant, and also slowly looses oil (no drips), however, there appears to be NO oil contamination in the coolant (which I have experience with and understand what it looks like), and NO coolant in the oil. I'm beginning to fear this is an "oh crap" situation.

Is that white smoke coolant or oil? Am I burning a small amount all the time, but only enough after a long coast to actually see it? Are we talking piston rings or head gasket? Would a compression test be worth my time?