For me additives are cheaper but is heavy oil better? Btw this is a low end flat head cheap push mower nothing special.
Yeah, I understand!
Yes, heavier oil is cheaper especially when changing the oil. Is it better? Well, today's oils are better!
Mowers generally don't require much oil to perform well. I mean, if oil is on the dipstick at all, your usually OK! I don't think a full crank case will help with the knocking. But, please keep is posted if it does.
I'm just babbling here but:
Lots of people buy the oil and then also buy the additive too!
Additives generally just make your oil thicker with very little benefit(if any) in terms of their own additives/ingredient package. Typically, they just don't have much in terms of their ingredients(additives). Maybe some extra Zinc? IDK!
STP, Lucas and the like, are really nothing special but thick(er) oils made from low(er) grade oils such as Group I. Whereas, today's oils are Group II, Group II+ & Group III. Which is why most of these products are labeled as "Snake Oil"(nothing special).
With these "Snake Oil" products, all your doing is making your oil thicker when you can just buy thicker oil and spend the money once. That's all I'm saying.
But yes I understand. If you are just adding some oil product to the mower's engine(not changing the oil) then, STP(or whatever) maybe the the least costly way to add oil volume and thicken it up as well.
Why the engine is knocking? Who knows!
Good Luck. I love free stuff!