That feeling of braking car is almost common with all vitz...
But The vibration in park mode might be a problem. Does it happen in D or N gear too? How about with and without AC? If it does only in AC then most probably it is not properly cleaned (fuel filter, spark plugs (change to OEM), trottle body, injectors, etc), better change your mechanic and do it by a professional... Use a company operated fuel station, and try adding some high octane fuel with normal fuel (1:3) though its only a temporary solution...
You can't use 0w30, but try using 0w20 if the engine is not very high on the odo, otherwise 5w-30..
