Gerami - hope you don't mind I split your gear oil question into a new thread.
I have experienced a situation where the owner's manual for a manual transmission car called for the wrong viscosity of oil compared to the oil that came in it from the factory. Using the replacement oil grade from the manual (a heavier oil) would cause the synchros to grind when the gears were changed quickly, yet the factory oil did not have this problem.
So it's worthwhile paying attention to these things & using the most appropriate oil.
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View my fuel log 2014 Mirage ES 1.2 manual: 62.4 mpg (US) ... 26.5 km/L ... 3.8 L/100 km ... 74.9 mpg (Imp)