OP I will give you the benefit of doubt and assume you meant change spark plugs at 5000 hrs and not 500---which is every second oil change give or take.
also it should be noted that viscosity is not the only property of engine oil that is important. viscosity has to be maintained at both room temperature and operating temperature, and this is accomplished by elastomeric molecules that get sheared by the rotating parts of the engine over time. also, other additives that control foaming, corrosion and wear get consumed over a limited time and given the chemicals lift trucks get exposed to, i do not like to push that envelope.
certain chemicals and especially water from stop/start condensation or outdoor storage will continue to dig and cause damage even if the lift truck is sitting idle, so even in low hour operation I dont think it is unreasonable to want to frequent the customer for check-ups more than once a year---a lot can happen to a sitting forklift in one year.