Its normal for engine to consume some oil, especially in this case of it being an older one with many other factors which we dont know like mileage, style of driving, etc.

How much KM travelled vs reduction is unknown.

Example the 1zzfe is infamous for having this issue (google it, class action lawsuit in US), yes, using heavier oil may slow down the consumption issue, but the core of the problem still needs to be identified/rectified, if the cost of repair can be justified.

Your mechanic is suggesting a troubleshooting step, which is a simpler one.

I've own 1zzfe with oil consumption issue, and many others who have this engine experience same be in an Altis, Wish, Rav4, etc etc. However there are no smoke from exhaust, or oil in the coolant. It is just the unfortunate design of the first gen 1zzfe, where the oil is slowly burned during the combustion cycle.