Just bought a used skoda with 61,000 miles. (auto) Previous records show its been using mostly Castrol 10w40 semi. Full service after I bought it they used 10w40 Quantum semi. Motul suggest using a synthetic X-cess 5w40. I am a bit wary about changing from semi to synth. Not usually a good idea. Should I stick with the semi or can change to the synth? Price is not an issue. Just want what is best for the engine.