I think every commercially available automotive motor oil has detergents. Grade SE or whatever is the latest standard.
Flat lobe can be noisy because there is now too much clearance in the valvetrain. If you adjusted out the clearance it might be quiet again but there is no point because the cam is shot.
I had a big block chevy once that had an annoying tick in the valvetrain that would come and go. I traced it down to a pushrod ball end that was off the pushrod centerline. Pushrod rotation in the guideplate would move the rocker arm around a bit and the tick would come and go. From this experience I always check pushrod runout and if one end is better, put that end near the rocker. Runout causes less problem at the tappet.
