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Regular or Synthetic shouldn't make a difference with modern oils as far as which you choose it is a preference. A harsh knocking is a sign that you are not getting oil to the top of the engine at startup.
What year discovery and how many miles?
Could be a clogged pickup tube or dirty oil pan. or the oil pump is not working well and needs repaired/replaced.
Also, what weight oil are you using? Did your mechanic change that too?