THe original complaint about synthetic oils and what they could do to seals was mostly due to the lack of parrafin (sp?) in synthetic oils which is present in regular oil. This compound was specifically attributed to regular oils ability to keep seals soft and pliable. Synthetics lacking this were causing worries as to the lifespan of seals, but I have literally switched 10's of dozens of vehicles to synthetics no matter the mileage and have yet to see any trend of leaking seals/gaskets that could be directly attributed to the oil. Age of seals/gaskets is 99.99% the cause of failure and any coincidence with the addition of a synthetic oil is just that, a coincidence. I have done extensive reading and find that most mechanics/engineers feel the same way.
PS: There is almost never a qualified person in an auto parts store with the proper knowledge of fluids to give any recommendations over what their computer screens tell them.
88 LX 5.0 Notch - MAF conversion/B303/Heavily worked GT40's/1.72's/HD FRP lifters/Edlebrock Performer/BBK 70MM TB+BBK EGR spacer/BBK CAI/19lbers/70MM Ford MAF/BBK Ceramic 1 5/8 LT's+BBK catted X+blowmasters/3.55's