From my humble experience with cars ( oil and fluids are cheap while engines and Transmission box are expensive )
The oil changing intervals are only estimates and recommendations based on the Oil companies and the vehicle manufacturers expectation of the driving conditions and driving behaviors of normal drivers on normal roads( which we are not ). so, it depends on how much are we pushing the RPM and redlining the engine and for how long and what's the temperature of the oil that we have reached all these factors combined will directly impact on the Oil characteristics degradation and will no longer be able to protect the engine components .
As my personal rule ( for only my off-road vehicle, for the other vehicles i use some more relaxed protocols 😂 ) i change the oil around 7 to 8k in combination with the following routine :
check the engine oil status using the dip stick once every 3 to 4 weeks ( or before a long IM drive like sweihan in AD) you will need to get your hands dirty and look for
- the oil level it should fall just below the MAX line ( if the oil is closer to the min , will have to top it up)
- the oil viscosity ,this is the most important characteristics of the oil ( if it is water like fluid then its about time to change )
- while checking the viscosity with your fingers try also looking for some solid particles in the oil if you do find ( change the oil immediately and flush the engine ) if some sand escape into the engine , this way you will save the engine before it get damaged.
- If the oil Smell has a burned like smell it means its about time to change soon .
- use only Fully Syntactic , as the oil which have the word "Syntactic " it means its a blend between syntactic and organic
- change the Oil filter with every oil change ( no cutting corners on this one sorry )
Very important to make sure to wash the engine bay before the oil change or at least to air blow and clean underneath the engine oil cap otherwise some trapped sand under the cap might fall inside the engine when opened !!!
While for the Gearbox, Transfer case , and differentials change the fluids between 30 to 40K . but if it happens and you drive in water for a long period of time without an elevated breather ( change all these fluids immediately after the drive )
For the airlifter i have and K&N oiled filter i knock the sand of it every 4 to 5 drives and clean it every 7 to 8 drives ( this might vary from one car to the other depending on the intake position in the car . as in my case i have a snorkel with a pre-filter
There are other things that needs maintenance like the engine coolant flush( usually 100K or 5 years ) break fluid and others ... etc you should follow the owner manual for your car ( changing these fluids will protect protect the bearings from corroding and will lengthen lifespan of these systems .