> Oil cooler O-ring for sure.  Always happens  when it turns cold unless you
> have new improved O-ring in there.
 fyi Part No. N 901 814.02
Technical Information
Group:    17
Number:   87-05
Date:     October 29, 1987
Subject:  Oil Cooler seal leaks in extreme cold
Model(s): Vanagon waterboxer, all Model Years

Extreme cold weather can harden and split the seal between the engine
block and the oil cooler thus creating an oil leak:
*  An improved cold-restistant seal was installed in production as of
   02/87.
*  The new seal can be identified by its silver-grey appearance and is
   available through your parts department, Part No. N 901 814.02.