A slight change in the weight of the oil shouldn't be catastrophic. However, manufacturers are very particular about which oils are suitable for their engines-much more so than the brand of fuel that the owner chooses to fill up with. There is a more detailed specification of the oil on the back of the bottle. Look at the API/ACEA ratings and compare them with the manufacturerer's STIPULATED (not recommended) specifications. In fact the label should specifically state which engines the oil is approved for. Ignore any pseudo-scientific babble about intelligent molecules bonding themslves to vital engine components!

If the Castrol stuff doesn't meet the specifications in the handbook, ask the mechanic to change it. Your engine needs the right oil, not a generic 'good quality' one.