Highway driving is less stressful on the engine (unless you are pulling a three hour stretch through the mountains of West Virginia at 80mph) and the oil has time to get hot and eliminate harmful contaminents it carries away. I would say that you could safely extend your change interval if you are doing mostly highway driving. It would be interesting to hear from anyone with FSS on this subject. Does the computer agree with me? Either way it can only do more good than harm to stick to your current schedule for oil changes. Change it hot and change it often.
1995 E420 - retired
1986 420SEL - retired