Cessna Skyhawk 172N Pilot Operating Handbook page 292

GEARBOX OIL
To ensure the necessary propeller speed, the engine is
equipped with a reduction gearbox filled with gearbox oil. (refer
to section 1 of this supplement for specification).
The level can be checked through a viewing glass on the lower
leading edge of the gearbox. To do so open the flap on the left
front side of the engine cowling.
The drain screw is located at the lowest point of the gearbox. A
filter is installed upstream of the pump, as well as microfilter in
the Constant Speed Unit. Check the gearbox for sealing after
the first 5 hours of operation (visual inspection). Regular checks
as well as oil and filter changes have to be performed in
according with the engine Operation and Maintenance Manual,
see OM-02-02. The Supplement of the Aircraft Maintenance
Manual has to be considered as well, see AMM-20-02.
WARNING:
CAUTION:
Page 6-2
Issue 2
Revision 6, Sep. 2014
Reims/Cessna (F) 172 N&P
It is not allowed to start the engine with low
gearbox oil level.
Between scheduled maintenance topping
up gearbox oil should not be necessary. If
low gearbox oil level is detected, inform
your service centre immediately.
Supplement POH
with TAE 125-02-114