small drop in oil pressure after switch to synthetic blend delo

[PETERBILL386]

hello everyone im a driver been driving since 05 oil field crude mostly now dry van otr just got a 2012 386 isx 15 with 482486 miles just changed oil last week from factory filled delo to delo synthetic blend and noticed right away small drop in oil pressure at idle fr0om 20 to now about 17 -18 so im worried did i screw up changing what i thought would be an upgrade ,little about the engine has has tube over valvecover so its an 870 right ?i have to idle alot almost everyday to keep batteries charged to run fridge and stuff it idles at 650 but then the very next morning like clockwork it idles up to 760 for about a 17min regen truck never runs hot i pull loads every week ranging from 10,000 lbs to 45,300lbs what would be any sdvice on keeping this engine going for as long as i can also nothing has been changed from factory sa far as emissions go its bone stock you know what i mean any help would be greatly appreciated thanks