Oh okay great, I thought you shifted from Swift, do not fiddle with the CVT oil if you believe the mileage is genuine. Just get in new engine oil and Oilfilter/airfilter. (in case you do not find genuine Toyota Airfilter do not settle less than guard filters. All others are crap.
Dont wash the engine compartment at all, Air wash is best and lastly no grease anywhere. specially the battery terminals.
PS: I get best mileage when i use Caltex Golden box, 5-40 you can use different oils as you like, lowest should be 10-30 Caltex red cap. (best oil for all categories).
Try to find last service stickers in your car, Maybe you can find out when was it serviced last.
Use google translator to know most of the Japanese translations about everything in your car,