The owner’s manual recommends thinner oils. Because car is new and most of the people consult with manuals to find that which oil is recommend. But we forget that these recommendations are usually for the new engines. And once the car is 4~5 years old and ran over 50K, then we have to decide by ourself that which oil suits more.
For new Car -- Use synthetic Oils. Mostly very thin like water.
Over 30,000 Km -- switch to 5/40 (presently best oil available in the market).
Over 80,000 Km -- switch to 20/50 and keep using it.
And if car is Diesel, then use 15/40, 10/60 or 20/60 after 100K.
If you car is driven over 100K and you use “Fully Synthetic or Thinner Oils”, it will produce extra noise in the engine. And you will feel the engine’s pick-up has significantly reduced (instead of increasing).
Hence switching to thicker oils is wise. But choosing the right oil manufacturers company is more important.
We know many companies using same oils, but they changed the packing and given it the new name like “improved formula”.
Shell, Castrol, Caltex, Total, Sinopec, Toyota, Havoline, PSO, etc. are old names to forget now. They are making money from our innocence and lack of informations.
Choose good companies like PTT- Performa, Mobil-1 (If original), ZIC, Redline (if original available), Zippex is also just OK.