andy you had the hyd tappets out? im pritty sure you can fit them in the wrong way on a 1.8 cvh so the oil fill hole doesent line up.
you did put them all back in the holes thay came from?
or did thay get mixed up
shouldnt take to long to whip of the rocker cover remove all the rocker arms and take out the tappets for a looky check the rollers on the bottom, oil holes, look down the tappet tubes to see if the cam is worn etc. even a oil wash out in the pariffin bath sometimes clears the sludge out and revives them.
failing that new tappets it is
too thick oil is just no good for hyd tappet engines as thay require a bit of leak by to lubricate and ajust corectly back in the early 90's i used to get engines running the cheap 20/50 5haging the cam lobes as the tappets failed to ajust going too tight and braking the hard skin on the cam all from the cost of a few quid on some oil
10/40 should be ok any lower could be a little thin for an engine of the cvh design era as thinner oils came later