Your mechanic is smoking the good stuff, after an engine overhaul you should use a thicker oil for the first 1000 kilometers only. This helps the break in process and allows for some wear and tear so that the life of the engine can be prolonged
It is a VVTI engine and it needs either 5W30 or max 10W40 grade oil, even toyota itself is using the wrong grade oil of 20W50 in pakistan.
Keep in mind that the oil grade required is mentioned in the owners manual and you must stick to it.