Confusion about engine oils!

Hey gang, I've recently got my dream 2000 7th Gen Celly, and after thinking I couldn't shift into fourth gear (auto) but I was apparently in non overdrive mode (still doesn't make sense to me), and not after driving it home in 3rd gear on the highway going high rpms for hours eek; I wanna change the engine oil. The thing I need advice for is the engine oil, I've done a good amount of research and I get different answers everywhere. And something I completely don't get is this chart in the owners manual, I thought the number before the W was how thin it gets at cold temp, and the next number is how hot it can get, but why is 5w30 so much lower max temp than 10w30 when they both have the same high temp number? I live in Queenland Australia where it's warm or hot most of the year and doesn't get much below 15 degrees in winter. Any advice or explanations would be so helpful thank you Cel family!

Hey gang, I've recently got my dream 2000 7th Gen Celly, and after thinking I couldn't shift into fourth gear (auto) but I was apparently in non overdrive mode (still doesn't make sense to me), and not after driving it home in 3rd gear on the highway going high rpms for hours eek; I wanna change the engine oil. The thing I need advice for is the engine oil, I've done a good amount of research and I get different answers everywhere. And something I completely don't get is this chart in the owners manual, I thought the number before the W was how thin it gets at cold temp, and the next number is how hot it can get, but why is 5w30 so much lower max temp than 10w30 when they both have the same high temp number? I live in Queenland Australia where it's warm or hot most of the year and doesn't get much below 15 degrees in winter. Any advice or explanations would be so helpful thank you Cel family!