I think there is nothing wrong with the engine, provided; engine health is good, compression is perfect, there isn't any smoke, abnormal behaviour, etc. Try switching the Shell brand, or the store from where you are buying the oil.
is semi-synthetic I believe! Try switching to fully synthetic, like Total, Kixx, ENEOS Sustina, PSO, Totachi, etc. and in premium brands; Amsoil, Kendall, Valvoline, etc.