They can be oil drinkers, which causes timing chain issues. Then the chain tensioner won't recover from being starved of oil. We just had an 08 STS in the shop for running poorly. 3.6, it was 3.5 quarts low and misfiring, rattling badly. Owner got lucky, added oil and half the issues went away. No leaks under the engine anywhere.Ya Iv heard in your opinion what are the common issues?