Shell Rotella does not meet Subaru's standards for engine oil. It also does not meet the API or JASO standards as recommended by Subaru.
In order to meet the tougher fuel economy standards all of the Subaru engine's that now require synthetic oil are built with tighter tolerances than previous engine designs and I would not doubt that using an unapproved oil could cause issues. Pay to put the recommended oil in the car and see if the problem is resolved. If it is then they are/were right.