I have 2007 A8 with 100,000 miles and I have the problem, coolant and engine oil mixed.
I realized coolant level decreased after I filled it up and I was shocked because engine oil looked like jam when I changed an engine oil and filter.

The coolant is decreased because coolant and engine oil mixed.
Is this a common issue for Audi?
Anyone had same issue and have a solution?

Please help me.

 

Start with a compression test first... if all numbers do not match then most likely you have blown head gasket. Either way those fluids mixing it means serious engine problem... and no its not common for Audi or any other car manufacturer to mix coolant and oil.
Did the oil looked like chocolate milk?

 

Agree with L_U_D_I_A on this.

Have only heard of it one other time, also on a 2007.
Search on AudiWorld for 'gel like oil filter' in the D3 forum.

Never heard resolution...

 

Thank you for your comment.
The oil looked like chocolate color and very thick.