When grease is the better fit
Grease stays at the lubrication point, limits leakage, offers some sealing effect, and can support long service intervals where continuous circulation is impractical.
When oil is the better fit
Oil is preferred when the system must remove heat, circulate through filters, reach multiple components continuously, or provide controlled flow at very high speed.
Use operating data, not habit
Compare speed, load, temperature, sealing, contamination, cooling demand, maintenance access, and OEM requirements before converting a point from oil to grease or vice versa.