Rental equipment fails on the job, in front of a customer, at the worst possible time. Preventive servicing is the difference between a planned two hours in the workshop and an emergency plus a refund.
Schedule by whichever comes first: elapsed time or usage. Ninety days or five thousand kilometres. Two hundred and fifty engine hours. Usage-based intervals matter most for the assets that earn most, because they age fastest.
The critical part is enforcement: an item due for service must stop being available automatically. A schedule that relies on someone remembering is not a schedule.