since youve tried all the oil stuff already have a look at the exhaust manifold,i had the same noise for months and messed aroud with oil flushes and filters thinking it was tappet related. Finaly turned out to be the
exhaust gas popping out of a tiny hairline crack right at the flange of the manifold,never thought exhaust leak could make that noise like tappets but there you go. might be an idea...