Thanks for the quick response. I normally try to buy shell or fina at high volume truck stops outside the DFW metro area. Occasionally I buy from an in-town fuel stop that sells the "reformulated" diesel. That fuel produces lower MPG in my experience, a possible tradeoff for reduced emissions. Perhaps the reformulated fuel also has an additive that degrades the o-rings? In checking my records,it seems that the leaks coincide with the fuel change, as you surmised.
In closing, what a great forum we have at our disposal.
Many thanks to all who take the time to respond.