When the manual says 7500 miles it usually says normal driving conditions, That being said very few cars are driven under what is considered normal driving conditions, around town and short freeway trips are severe driving conditions thus needing much more frequent oil changes, Ashman has it right 3000 to 4000 miles should be the maximum miles driven between changes no matter what kind of oil you use including synthetics. oil is way too cheap not to change it often, I'm sure Mercedes owners follow this rule but jap and US car owners are notorious for not doing maintenance untill something goes wrong.