Take off the intake (lower) and valve covers. Take off rockers, remove pushrods (inpect for straightness), inspect lifters for missing clips etc or wear, ck those cam lobes while your at it (5.0's ussualy dont wear much from the rolers) and change the lifters. Additives? Water in the intake that I know works to break up carbon and reapiar carbon knock. Oil additives usually and I sau usually so I don't start a flame war tend to swell and eat up gaskets due to the cleaners and chemicals they are composed of. Say like diesel or keosene (yeah alot of companies use it) put a o ring in some of it and watch it swell. Anyway when you get the lowr off you may have sludgebuildup under the lowr in take and in the lifter valley scrape it clean it spray it with brakekleen after all is assembled drain oil relly well change oil. And enjoy another 50k on your 5.0. My 86 lincoln went 255K before I sold it yes sold. With just oil changes and at 166K needed an oil pump.
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