Ruh Roh Rotella...
Sorry, you might be out of luck, as new 2011 WRX/STI manuals say this now:
And the 2011 WRX/STI Quick Reference Guide says:
You might have an older 2011 manual that says 40 weight oil is OK for high ambient temperature use, but Subaru should have sent you an owner manual addendum correcting that to the current 5W-30 SM or SN Energy or Resource Conserving, ILSAC GF-4 or GF-5 requirement.
This is why I'm avoiding Rotella T6 for the time being and trying out the new Castrol Edge Titanium/Syntec oils. Many other members of this thread love Rotella T6 (supported by great used oil analyses) but I always suspected something like this might happen.
Also if the oil change does fix your misfires, then you do owe the dealer $110. ..but if it doesn't fix the problem then I would withhold payment until they actually fix it.