Seems like you specifically created an account on Pakwheels to find a remedy...hope you have read issues related to JDMs mostly coz of low grade or fiddled cars
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In response to your query....
Regardless of ODO mileage at the time of purchase, these JDMs Engine Oil, Oil filter, Air filter may needs to be replaced. Afterall, these are whatever kms driven cars, imported and could have stayed on port for a while, various climate conditions, etc, etc. So be careful to trust whats came with it, get along with someone who have similar package JDM and get regular maintenance items checked or replaced.
What most dealers do is, change items which are rather not apparently checked by customers. Your JDM car variant should atleast maintain 16 to 18kms per liter.Fuel do matter, try a well-known fuel station around,but, High-octane is not necessary.