Pontiac G6 P0011 Code

I am assuming you have a 2.4L 4 cyl engine. The code is related to the camshaft timing, they have a variable cam timing system and the correct oil is critical. GM does not recommend any additive like what may be present in your high mileage oil. Also you did not mention the viscosity? GM recommends 5w30 only. Hopefully the correct oil will do it but it may also be caused by excessive timing chain slack a bigger problem. It is less likely to be the actual cam shaft sensor but still possible and easier and cheaper than doing a timing chain.

From your description, I'm going with the oil viscosity as acauto suggested. It typically is not the type of oil (synthetic, blend, etc) but make sure whatever you use complies with GM's viscosity and API recommendation.