Your symptoms are classic. They almost universally describe the syptoms of bad valve guide oil seals. It also is classic for a bad diaphram in the vacuum modulator of the automatic trans.
What makes this the answer is that during the drive downhill manifold vacuum is the highest ever, often reaching 20inches or more. This sucks the oil in through the guide seals or diaphram.
I know of no such a case involving coolant.
Bosch Master, ASE Master, L1
33 years MB technician