When you "topped off" the expansion tank, did you loosen the bleed screw first? The 6s are more bleed-fussy than the V8s. I had a friend ruin the head gasket on an E34 520i (living in Europe at the time), because on a long drive he "topped off" and then drove in the red for several hundred KMs thinking there was coolant in the system. As soon as we loosened the coolant screw, it was obvious that his coolant system was full of air. Alas, too late; the white smoke and antifreeze in the oil appeared a short time later.
`97 540i 191K