I've been using Mobil 1 for years and never had blacking or coffee looking problems in even high mileage cars, but I change my oil in all cars no longer than 3K miles. I have a wife and 2 kids who beat their cars and in the heat out here it's the least I can do. I am listening to everyone on this though because it seeems to be a consensus that Subaru boxers and Mobil 1 oil don't like each other...curious though because Porsche uses M1 as their preferred oil. Regardless I'm not hear to argue.
Has anyone looked at Eneos Sustina? I used their stuff in Japan and a lot of you look for specs and info and they place it all out there...even the much talked about Zinc chatter and they don't add sulphur. Most of their dealers are in Cali and they are Toyota and 1 Subaru dealer...that's a good sign to me.
My 2 cents...I will say the regular Full Synthetic Eneos is about $54 a case...Sustina is going to be on par for price with Ams oil and others I suspect. I only know Yen price.
http://www.sustina.us/superior-cleaning-power.php
http://www.sustina.us/ultimate-fuel-efficiency.php
http://www.sustina.us/dealer-locator.php