Sentra, sorry to throw a monkey wrench in to your argument, but look at
http://priuschat.com/attachments/toy...ons-pdf.49252/. That's a Toyota oil application chart for Canada, same cars and same engines as the US, yet Toyota has okayed use of 20 weight oils there for years. 1998 is the year the N. American Corolla began production at NUMMI using the US produced 1ZZ-FE engine, that engine was designed and tested to use either a 20wt or 30wt. Also, the 1NZ-FE engine used in the Echo and Yaris was adapted from the 1NZ-FXE that was designed for the original Prius and has always used 20wt (1NZ-FXE). The only difference between the 1NZ-FE and FXE is cams and pistons, all other parts are identical, as verified by part #s, including all bearings. In Canada and other countries, the Echo/Yaris has always been able to use 20wt. It's Toyota US that has disallowed the use of lighter oils for warranty purposes, unless they say so. If my car wasn't still under the powertrain warranty, I would be using 0w or 5w20. Thick oils don't necessarily protect better than thin oils, the important criteria is film strength, which is independent of viscosity.