I went to my trusty oil changing service (have been using them for years with my wife's 4runner) and had my first oil change (did this at 2000). I requested synthetic oil as well. -Well this morning (2600 miles later) my check-engine light went on. The dealer said the error message reports an oil pressure problem. I checked my oil change receipt and it appears 5-40 wt had been used at 2000mi, not 5-20. Even if i knew this at the time I wouldn't have blinked. -but the dealer tells me even 5-40 is thick enough to cause problems. So.. it appears I'll be going to the dealer for my oil changes from here on (even though they aren't carrying a 5-20 synthetic). The 5.7 is very particular.