what specific oil to use?

[dags]

i have a 1hd-t engine with 205,000 kms on the clock. a mechanic told me not to use my existing oil (mobil delvac 1 fully synthetic) on my next change oil becuase,

1. oil turns into plastic because of our weather condition btw im from the philiipines

2. not applicable to our weather condition

3. as the clock goes higher you have to change the viscosity of your engine oil??

what should i do? should i change the viscosity of my oil? help! help!