Well you can rule out the fly wheel not catching. You'd hear the starter spinning about a million RPM if that was happening. Also, rest assured this has nothing to do with the oil or any other fluid.
Please explain how you tested your battery and starter?
There are a number of fuses and relays that apply to the starter.
In the under hood power distro box:
Fuse #5: 50amp ignition switch and starter relay. If this is blown, you won't get shit.
Relay #6: Starter relay. If this is shot, it will not crank. This relay is not
the main starter relay. It is actually only a relay that switches the big relay built onto the starter motor itself.
In the fuse panel inside the cab
Fuse #24: Clutch position, starter interrupt, and anti-theft (all if so equipped). I don't know how any of that shit works. But if the fuse is blown, it might screw you up.
Then on the starter itself is the big ass real starter relay.
To engage the starters relays, the power flows like this:
Battery --> Power distro box ---> 50 amp ignition fuse (#5) ----> key switch ---> power distro box starter relay (#6) ----> Starter built in big ass relay
You will also have a large red cable going from the battery to the starter's integrated relay. This carries the power (very high current) that actually runs the starter motor itself. The big ass relay built onto the starter motor switches this power on and off. If the clamps on the battery are corroded, it will inhibit the flow of electricity. Same with the connections on the starter's integrated relay. This is a pretty common cause for no-start. Disconnect the battery clamps and clean them all nice.