Kia Optima 2017 Owner's Manual page 622

Maintenance
NO.
Fuel filter cartridge
*7
(Diesel)
*8 Manual transaxle fluid
*9 Valve clearance
*10 Spark plug
8-36
ITEM REMARK
This maintenance schedule depends on fuel quality. It is
applicable only when using a qualified fuel <"EN590 or
equivalent">. If the diesel fuel specifications don't meet the
EN590, it must be replaced more frequently. If there are
some important safety matters like fuel flow restriction,
surging, loss of power, hard starting problem etc., replace
the fuel filter immediately regardless of maintenance
schedule and we recommend that you consult an author‐
ized Kia dealer for details.
Manual transaxle fluid should be changed anytime it has
been submerged in water.
Inspect for excessive valve noise and/or engine vibration
and adjust if necessary.
We recommend that the system be checked by an author‐
ized Kia dealer.
For your convenience, it can be replaced prior to it's inter‐
val when you do maintenance of other items.