I have a 1989 535, Mine does the exact same thing. Burns coolant every third or fourth blue moon. I cannot replicate it. Does not over heat, does not mix fluids. I have even given up on checking coolant because it doesn't ever seem to go down. The coolant resevoir isn't gunked up either. The car runs fine. no misses, I have had this issue sense Ive had it(3k miles now) Its a head gasket going really slow. Mine is cylinder 6 with the issue. I have had my car to 100 many times been to 120 once. I don't know how to blow it lol
Theres only two ways to burn coolant, cracked head or a gasket.
I would say your safe to drive it until your ready to do it. But I would never put off a head gasket if I cared about the vehicle.
READ RADIAN'S WRITE UP ON HEAD GASKET. It is amazing very well documented, And honestly if you care about your care thats how it should be done.
2003 GMC Sierra SLT 2500HD CC/SB