I have always used Mobil 1 10w-30, but recently I went to Mobil 1 10w-40 High Mileage. For the hot days coming up, will 5w-30 be best?

 

5w-30 is recommended for less than 60 degrees in the owners manual.

 

That's what I run in my V10 year round.

 

Synthetics have a wider temp range.

 

Hell no it wont hurt. I run 5W-30 Synthetic in the winter. And 10W-30 Synthetic in the summer

A different grade of oil wont hurt these motors. Maybe a bomb, but not oil.

 

I was just saying why would he switch from a summer oil to a winter oil when we are going from winter to summer?

 

In the winter sure, but in the 100 degree Texas summer?

 

...Run a regular 10W-30 oil. There is no need to use a high mileage oil unless your engine is leaking oil (plenum gasket does not count).

 

Re-read my post. Right now you are using a thick oil. Why would you switch to a thin oil for the summer? You run a thinner oil in the winter so it flows better in a cold start. In the summer you don't need that as it is warmer and the oil is already thinner.

 

This is what I use. It was designed for trucks.
http://www.mobiloil.com/USA-English/..._SUV_5W30.aspx