the dealer i went to last time used 15w40 for some reason, whilst this dealer i went to recently poured in 10w30. as i've stated above, the grinding sound and vibration didn't occur whilst the engine was on 15w40 oil. your theory is valid, and i am unanimous with it by all means, however for some reason the situation doesn't seem right. did u actually see the oil and the label? maybe u should give them your own oil next time u service your car there. becareful of the thieves though.