Are there any adverse effects of putting 15w 40 oil in a 5.2L V8?

What's your mileage? Why do you think you need to go to that? I have 180k on mine and some knocking so that's why I use it. (Has helped only a little bit.)

does more harm than good on startup. The first number is how the oil reacts at 32 degrees (like a 15wt standard) and the second number is the operating temp wt (45wt oil). Thick oil has reduced flow obviously and reduces gas milage and can accelerate wear on your oil pump.

the main reason i'd use it is b/c its free i only have 95000 miles i was just lookin for an easy way to save some bucks. i have a slow oil leak i can't find so i'm always buyin more.

its up to you...just stating what i have heard and some POSSIBLE side effects. Could go worry free and free oil changes certainly offests any small mileage change.

what weight i recomended.my oil fill doesnt have recomended weight on it so ive bwwn using 10/30

It will be fine.
Kinda thick for winter but fine for spring, summer, etc

whats the best oil for me to use in a 5.9 gas wit 184k on it theres no knocking or nothing?

any modern oil with the required API ratings is good oil. its more a matter of opinion and preference than anything. i've used havoline 10w30 or 10w40 for many years and a lot of miles.

What adverse effects? None so far in over four years. Motor just turned 100k and will burn thinner oil. Works great for me.