I just finished up a head gasket replacement on my 87 745t, the normal symptoms of a bad head gasket (except for the slow coolant loss) were almost non-existent right up until the end, after I ran the engine a bit warm upon blowing the heater control valve. The coolant had always dropped slowly since I bought the car, even after a water pump and hoses change, with no leaks detectable. I think a bad reservoir cap helped things last as long as they did, the cooling system had quite low pressure this summer; after I replaced the heater valve, tank and cap things really let loose under the higher pressures.
At no time, however, did coolant enter the oil or vice-versa, once it gave up the ghost for real and started dumping coolant in earnest it was all leaving the engine directly through the cylinders. The post mortem on the old gasket confirmed this, the oil passages seemed intact but a couple of the coolant passages, and the area between cylinders 2 and 3, were completely destroyed.
I hope you get away without doing the headgasket, however it really is not an impossible task; on an 85 you must be just about ready for one anyhow... It is also a great chance to go over everything on the block to check for any leaks, oil box, engine mounts, whatever-- with the head and intake out of the way everything is really easy to get to. Parts are cheap and if you can afford a little time it is effort well spent; my brick has never ran so well or been so smooth.
Best of luck with it,

Shifted
brickflogger@aol.com
--
'87 745t 205,000