I'm no kind of oil expert, But with the hot weather lately, would not our motors do well with a thicker oil, such as a 10-40 wt???

 

That would give Infiniti a reason to deny any warranty claims, if something were to happen.

Regardless of ambient temp, your engine's temp should (theoretically) always be in the same range via the cooling fans. You don't want your engine to be too hot or too cold.

 

Are our motors doing poorly?

Not all 5W-30s start off thin. Some start off thicker.

Further, even if you use a thin 5W-30 oil and are worried about your oil shearing to a 20-weight, there are oil additives you can use that will stabilize the oil and preserve the viscosity. I've read where Terry Dyson recommends Schaeffers #132 for this purpose.

 

Better off using a synthetic 5w30 that can withstand higher temperatures.

 

Not necessarily. Either way can work.

 

Sorry -- should have been clearer. I was responding to the OP, not your post specifically.

I meant better using a synthetic that can withstand higher temperatures instead of using a 10w-40.

 

Right on, agree totally. Use a superior oil and get superior results
without presupposing improved chemistry by mixing two very
dissimilar products. If it worked well and it was that easy, more
than one expert would be advocating it.