[SentraSE-R]

They're your engines, but consider this: The bearing smoothness and tolerances on a 2006 Corolla or Yaris engine built for (and/or tested with) 5W-20W viscosity oil are likely to be much better than the bearing smoothness and tolerances for the 2000 versions of the same engines which were built for 5W-30W or 10W-30W oil. Also, we're not just talking about cold starts. We're talking about operating your warmed-up 2000 Corolla engine designed for 30W oil, hoping to lubricate it with much thinner 20W oil. If Toyota's engineers thought that would be OK, would they have said "DO NOT use these oils in engines other than those listed above. These low–viscosity oils cannot maintain lubrication effectiveness in engines with older designs and could result in smoke emissions from the tailpipe and/or unusual engine noise?"

"smoke emissions from the tailpipe and/or unusual engine noise" is engineer-speak for irreparable engine damage, IMHO.