Hi Joe and welcome aboard!
Yes you're Toyota Corolla was factory filled with synthetic 0W-20 motor oil. Using petroleum oil, fuel economy will be reduced, emissions increased, cold winter engine starts will be harder and there could be increased engine wear, engine operating temperatures will increase, hot engine protection will be reduced, sludge and varnish may accumulate inside the engine and maintenance costs will also increase.
They had you sign a waiver for the warranty because your Toyota engine requires a 0W-20 synthetic motor oil of the correct API motor oil grade in order for Toyota to cover any internal engine warranty claims and the quick lube didn't want to be held liable for voiding your Corolla's engine warranty. If the incorrect oil is installed in the engine, (as it was in this case), and something inside the engine fails due to the incorrect oil being used, Toyota will not warranty the engine. That can put a dent in your wallet to the tune of thousands of dollars for a replacement engine.
I'm an independent AMSOIL synthetic lubricants Dealer and the below link will open to my online Product Application Guide which will provide you the correct viscosity oil to use in your engine, all fluid quantities, filter part numbers, etc. This is absolutely free to use.
Online Product Application Guide
http://www.amsoil.com/mygarage/vehiclelookup.aspx?i=1&zo=349698
Yes you can change back to the better performing synthetic oil with no problems. You'd be very wise to do so.
$100 for a synthetic oil change seems very steep. An AMSOIL synthetic motor oil change costs far less money than that and my customers pay about $42 or so for their Corolla's to move up to AMSOIL quality and that includes a Wix quality oil filter, along with sales tax delivered right to your door. I think I can save you a whole bunch of money.
Send me a private message. Below is a 5 minute video on the basics of synthetic oil and what makes a synthetic oil so different and superior to petroleum oils. Enjoy.
