94 Full size 5.7 L with 206,000 miles on it. The book calls for 5W30 and wondered by it calls for that thin of a weight. I'm running Castrol Syntec Blend and wondered would it hurt anything to run 10W40 in the summer and 10W30 in the winter and if anyone else is doing that or sticking with the 5W30.

 

I've always ran 10-30 all year round. But I do know a lot of people who do that though. Thicker in summer and thinner in the winter

 

Either 5W30 or 10W30 (unless you live in a very cold climate) year round will be just fine. Here in Texas I run 10W30 all year.

 

What is the motivation behind trying to change the weight of the oil to a 40 weight?

 

With miles getting up I was worried about things getting loose inside and wondered if a little heavier oil would be better especially in the summer heat.

 

I lived in MN and CO for a while and did just that - thicker in summer, thinner in winter, never had a problem. I'm wondering what is the reason why some say do not change viscosity - what does it do that's harmful?

 

Switching between 5W30 and 10W30 would not be a problem. The issue with going to a 40W is that the oil pressure is controlled by the restriction of the oil passages within the engine (not by the pump, as some think). The heavier SAE grade that you use, the greater the pressure due to flow restriction until you reach the level of too high pressure and start causing problems, plus the delay in initial flow restriction at start-up.