Using 5-30 wt Oil a bad thing in '09 4Cyl?

having the oil and filters for my Wife's Highlander on the shelf, I just used them before I noticed that the CAP says right on it: 0-20 / 5-20Wt. Dhooh !! Is this really a problem here in a warm climate? Hasn't blown up yet.
What is the reason for such skinny oil anyway?

 

Won't be a problem but just switch next oil change. The light oil gets better fuel efficiency.

 

The owner's manual says 5W-30 is ok, recommended for heavy duty use. That is what I use in mine. My gas mileage beats most on this forum--always above 25mpg.

 

I changed mine to 10w/30 high mileage Mobil 1 at 130,000.figured it couldn't hurt

 

Thanks for the response. I will likely change at the next replacement..... maybe. Anyhow, I am new to this truck, and cut the oil filter open to see if there was any surprises from the last owner, looks OK in there. This 5 year old truck has 28,000 on it, last guy was 94 when he stopped driving by his own decision. He gave me the truck,.....Pretty darn nice! I don't want to screw it up.