There are a number of things that can cause oil in your coolant. In this video, I will show you how to check the engine oil cooler on a VW along with many other tips that will help you find out how oil is getting in your coolant overflow tank.
Oil coolers and transmission coolers are very comment causes of oil in the coolant so you want to rule these things out before suspecting a blown head gasket problem.
VW oil cooler: https://amzn.to/2Gqm4lG
Head gasket tester: https://amzn.to/2IiYqUo
27mm socket: https://amzn.to/2pSGHMG
My video on checking for a blown head gasket: https://youtu.be/wVDsreabEr8
The one tool every mechanic should have: http://amzn.to/2FN2r70