if hydraulic will have been filled with thrashed up petrol diluted oil in all its foamy goodness.
it will take a wee while to move the air on through.
or you have washed a nice slug of sludge in there
it too will take a while to work its way through
if it continues after a full operating temperature drive then i'd be a little more concerend
but then again all of mine do it for the first 15 hours of motoring so my recently accuired blase attitude to this kind of thing may not be a good example to be setting

if it drives alright and your oil pressure is good give it a week or to and see how ya go.
yes you can damage bearings and lifters by running when the oil knackered by petrol or past its sell by date but you have to try quite hard
decent pressure you'll be fine and 20W50 is the way unless you need something special for racing and then special racing 20W50 is probably the way...materials used and engine design dicatate this to a certain extent
my 6 does not like standard multigrade your bigger v8 won't either
Dave