motor oil and oil pressure

[Keywest18]

well i just changed th oil in my blazer for the first time since ive had it about 2 months.and i noticed before my oil pressure fluctuated a lil bit never really paid attention to it to much.well ive been readin and for high mileage vehicles people were running synthetic(ive always used conventional 10w40) well i went to walmart and got some synthetic 10w30 Q state.well now at idle my psi is at 20psi but driving and over 1000rpm its at 40.my oil light doesnt come on and i dont think its that big of a deal but i just want some reassurance as i know oil and oil pressure is very important.also what oil do you guys use and why?