Absolute capacity is 5.2 US qts.; You don't see that much come out or go in after an oil change though.
http://www.cygnusx1.net/Supra/Librar....aspx?S=A&P=35
Are there any aftermarket components on the vehicle which may account for the higher capacity? Oil cooler, an accusump perhaps?
This belongs in the MKIII Generation discussion forum, I'll move it there.
Is the stick still relatively straight? The tube doesn't enter at oil level and a significant curve may cause a low reading.