Its totally silly.
someone working in Toyota does not mean he is well qualified for suggesting the actual fluid for your vehicle. does he know how to read the manual? these people are simply innocent and just doing their jobs which they are supposed to do.
many years ago, when i bought my passo clocking 75K, i took it to Toyota and they poured 20w-50 Petron and said this one is best & recommended for toyota. It has amazing results and etc. etc.
But i immediately start studying at PW forums and very soon i knew that 20w-50 will ruin engine in coming years. So i immediately changed to 5w-20 (Zic A+).
if you can type the message here in English, that mean you can read & search too.
Find the manual and read it. Or read the millions of previous posts here at PW Forums. After that you will literally go back to that mechanic of toyota, but this time not to change oil, but to hit his butts.
As far as engine life & thinner oil choice is concern, HHHMmmm......... let me tell you that my Crown 3000cc Petrol engine is clocking 380,000 KM. and i am using 5w-30. Earlier i was using 5w-20 and engine is strong like Bull.