Oil doesn't know the passage of time for the most part, so unless the environmental or operational situation is very poor, one, two or three year time limits are perfectly fine. There are numerous UOAs here that illustrate this, including some on 20 and 30 year old oil that was usable and is being used. If the vehicle is stored in a dry, relatively temperature stable place and when run, it gets run long enough to bake out moisture and gets good and warmed up, it's fine.
I run an Honda and a late model F150 at 18 month, mileage only, intervals, farm tractors and a diesel pickup at two and three year hours-only intervals and have proven them all via UOA. One of the farm tractors is usually on a three year interval and sits 3-4 months at a stretch in an unheated barn. UOAs are fine.
You can find numerous other testimonies here of people who run long time intervals. We don't always chime in because this questions come up on a monthly basis and we often get tired or repeating the mantra that is already here to be found and read.
Most people do more frequent oil changes than they need without really knowing why, either out of habit, fear, or being influenced by sales-based misinformation designed to get you to buy more product. You shouldn't extend your interval in the same shroud of ignorance either. It should be based on some study and, again, that comes down mainly to the environmental and operational situations. It's best to back it up with a UOA too but that may add to the financial aspects.
If that '91 truck is short hopped most of the time (under five mile runs) and never gets fully warmed up, six months is appropriate, though it might likely still be able to do a year. Just about any oil can do a year, even in fairly poor situations. If it's driven and stored right, just decide on a mileage interval and change it when the odo clicks to it. If you want to verify, do a UOA at that point, one that has TBN and TAN at least, and verify. If the numbers look good, go longer if you want.
