@ Heart_Break_Kid
i've used 5W-30 Fully synthetic by Caltex Havoline in my 5 year old Corolla SE-Saloon Auto (without CNG)
at was not as old as 10 year old car but i've faced number of problems,
> Noisy and tic tic sound from the engine,
> noticeable decrease in mileage,
> Superior Pick but noisy start up
I reverted to normal 20w-50 just after using the synthetic oil for 3000 km, also had to change plugs and some other adjustments to make it normal though it is still a bit noisy and i might go for tappet adjustments next week.
based on my personal experience i do not recommend synthetic oils for any cars which is 5 or more years old, you can easily face the above mentioned issues or engine deterioration will increase dramatically.
btw I've also used Mobil 1 5w-50 Fully Synthetic in my 2008 model Vitz and i'm more than happy but still engine is bit noisy than normal, rest is fine cos it is not that old engine!