Oil deposits in coolant expansion tank

[Pablo Carselle <pcarselle@worldnet.att.net>]

Dear Audinats:
I hope what I fear is not right. I own an 875KTQ, 148K miles and was in 
the process to flush the cooling system and add Dex-cool antifreeze, 
when I discovered what appears to be oil deposits on the sides of the 
expansion tank. I stopped cold and began making some assumptions, most 
of which I hope  are wrong. So, here it goes.. car runs perfect boost 
1.6 to 1.7 maybe more but I don't push it, all systems work etc....
What is your diagnostic ???
Thanks in advance (TIA)
Pablo
87-5KCSTQIA1
85-5K Auto
90-100 Auto