A friend of mine owns an Audi A4 and he put Castrol GTX 20W50 in it because that's what he always used. One morning it was 20 degrees in Dallas. He started the car remotely in his driveway for it to warm up. When he went to leave the engine wasn't running and it wouldn't start. It cranked like the timing belt was broken. Since it was under warranty he had it towed to the dealer. The service rep asked him what oil was in the car and he told him. They changed the oil and all was well. The thicker oil had pumped up the lifters just enough to keep the valves open. The dealer had several other cars with the same problem.
I use Castrol GTX 20W50 in my M42 whether Audi's like it or not. It freezes at least once a year here but the winter nights usually average 45 degrees.
