That is a very interesting theory. We had been using Amoco
for both the E and the C without any problems, however the station near our hous was closed for underground tank replacement so I filled up at a Shamrock station. The gauge failed the next afternoon with the gauge pointing to empty and the light coming on intermittantly. I have an appointment with the dealor to look at this and change the oil as we are within 2K on the ESS and I would like to change the oil earlier anyway. I will try driving the heck out of this weekend and refilling it with our normal fuel.
I had run the E on Shamrock gas for several years without problems but this certainly seems to parallel your experiance. I will keep you posted.

__________________
Dave
88 300E 250K Sold
2000 C230K Totaled @104K
2003 ML320 156K Sold
2009 E350 4Matic Sport 46K Sold
2011 ML350 60K Sold
2014 GLK350 46k Sold
2012 GL450 67K
2016 E550 Cabriolet 48K