i have a 2006 porsche cayenne s it has 35000 miles i bought it used when it had 25000 miles from the dealer who did an oil change at that time. i was told it needs an oil change now that it has 35000 miles by the dealer at a cost of $275. do i have to use the dealer or can i go elsewhere and do i need syntheyic oil. by the way the oil on the dipstick looks preety dirty. thanks kent