Use Mobil premium. It will clean out everything. The knock you hear might be from dirty lifters, especially at this age. There was a secret recall on the cams in 305 engines made from 1975-1983. You might check with NHTSA web site for recalls on yours. If your engine has a lot of sludge in it, use a de-sludging chemical to restore lubrication to all your expensive lifters. Best one is "Power Tune" from Mercury boat store. Just spray contents into valve cover, drive gently for 2 days, then change oil to Mobil Synthetic 0W-40. drained oil will look like street tar. The foaming action from the butyl cellosolve (active ingredient) will have removed all the oil contaminants and your lifters will be quiet again!!! E-mail me back with your results!!!