Happened to me a few weeks ago.
Cruising at (too much) MPH on a very hot day, a cloud of white smoke appeared in my rear view mirror. it was not a cloud, it was a storm warning.
Slowed down, way down, got the next exit, crawler to the nearest service station.
Transmission cover gasket bolts worked themselves loose (still can't figure how this can happen.)
Oil was getting out of the transmission between the gasket and the oil pan and right on the exhaust system.
Tighten everything up, check oil level (low by 1/2 quart), back on the road
No more white cloud.