Yes — severe-service intervals apply the moment you leave pavement. Dust, low-speed high-load crawling, water crossings, and long idle periods all shear oil and load the filter faster than highway miles. Most manufacturers already list a severe-duty schedule; follow that one. If you wheel hard or tow, change on engine hours, not just the odometer.

Full synthetic is the safer choice for anything worked hard. It holds viscosity at the high sump temperatures that crawling and desert running create, and it flows better on cold mountain starts. The cost difference is trivial against the abuse. The only reason to run conventional is a specific engine still under a warranty that requires it.

Differential and transfer-case fluid, mostly. Bigger tires and steeper gearing raise driveline load and heat, and water crossings can contaminate axle oil through the breathers. Refresh gear oil more often than the factory book suggests, extend axle breathers above the waterline, and don't forget power-steering and brake fluid on trucks running heavier steering loads.