You can check to see if there has been any elongation of the timing chain by setting your crank to TDC for #1 cylinder and then see if the timing marks on camshaft sprocket and on the cam tower line up while at TDC. Look for scoring or other wear marks on the chain. These double-roller timing chains built with high-quality German steel and machine work, in my opinion, do not have a so many miles and replace tagged to them. If the oil has been changed regularly there is really no reason to change them until they show signs of trouble (e.g. noisy, hard to start (because of chain)), etc. I'd go with your mechanic's advice. Save the dollar's of installing a new one and shorten your oil change interval. My bias is towards full synthetic lubricants.