change engine oil to 5W-30 and after 1000 km visit a workshop which can perform compression test.
1000 CC 1KR-FE is notorious oil burner. but 1300 SZ engine is fine. It is likely sludge formation due to bad oil grade usage which is reducing oil.
change oem oil filter when changing engine oil grade. , please only USE a oil grade from brand fuel station do not fall trap to imported names.