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Well again, driving conditions and driving style have a lot to do with it.
Retired or not, at least change it annually. Oil accumulates lots of dirt (the stuff which passes through the air filter - a significant portion of which are silicates, very hard and able to create significant wear), moisture and fuel (combustion by-products). This is precisely the reason you change it at regular intervals, not because it's lubricating properties have failed.
In a sealed system, a quality motor oil such as MB1, Royal Purple or RedLine could go maybe 50k mi. without a dangerous drop in lubricating properties. This is partly why the tranny oil has a much longer life - it operates in a sealed system. Unfortunately, the engine is not a sealed system.
If you don't respect the equipment, or have short term ownership goals, it's certainly not as important. But, I've never heard of anyone having maintenance or wear issues from changing the oil on a regular basis either annually, or mileage based.
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