Tim, seriously doubt that the synthetic oil vs. mineral oil has any effect on the oil pressure; however, the weight oil you are using may have some effect. Factory fill for that vintage engine was 15W40 mineral oil. I run Mobil 1 15W50 year round in our 1995 E320. You may want to run the heavier weight synthetic oil in the summer months, if not all year.

__________________
Fred Hoelzle