> Would it be a bad idea to put in 0W-30 oil in my '99 Camry?
> Without an oil pressure gauge on your dash
> you wont know if its too thin.
Even if you had a gauge, I doubt you would notice any difference. When cold, the 0W30 is more viscous than "hot" 5W30. When hot, both have essentially the same viscosity. So unless you are starting up at temperatures around freezing and you are paying very close attention to your initial oil pressure readings, you are not likely to see much difference in oil pressure readings.
I can think of several reason not to use 0W30 -
1) If you are not in a very cold climate, the difference in viscosity between 5W30 and 0W30 at normal start-up temperatures is trivial and will have no effect on engine wear.2) 0W30 will cost more and there are no significant saving that will offset this cost increase.
3) 0W30 conventional oil MAY be more likely to breakdown than 5W30 oil. Given Toyota's history of sludge problems, I wouldn't take a chance on using an oil that MIGHT be more prone to sludge formation.If the OP is really concerned about the quality of oil he is using, then I'd suggest using 5W30 synthetic oil (real synthetic oil, none of the Syntec or XL7500 faux synthetic crap).