A 7.3 Powerstroke is about 3.5 gallons with a filter change. Make sure the 5 gallon bucket is empty when you start...
In general, "annually" is probably fine for most vehicles driven less frequently, though if you're never getting them hot, you might consider doing it a bit more often.
The major failure modes of oil, as I understand them:
- Just getting dirty. Oil filters help remove some stuff, but they're normally full flow (all the oil from the oil pump flows through them unless the bypass valve is open with very cold oil), so they can't be tight enough to remove everything and still supply oil to the engine in sufficient quantity. For a lot of larger engines, you can get "bypass oil filters." These take a fraction of the flow off the oil pump and run it through a far, far finer filter before returning it to the oil supply, pretty much solving the particulate issue.
- Water in the oil leading to corrosion. If the vehicle is mostly used for short trips, this is the most common problem - you end up with condensed water from the atmosphere in the oil, and it attacks the engine. Getting the engine up to full temperature, and holding it there for a while, mostly boils it off (think 45 minutes on the highway). I think water in the crankcase ends up acidic through a variety of means, which isn't good for the metal.
- The various additive packages wearing out. There are all sorts of additives in oil to help with various things - buffers to help keep the acidity down, zinc and other metals (not anywhere near enough of it anymore, because it fouls catalytic converters, use diesel oil in old engines) to help protect metal if there's metal contact (it forms a more or less self healing layer), etc. These just get used up. If you've got a large engine with good oil monitoring, you can actually get the various packages to add back in as the materials are used up, but this just isn't worth it for most people as opposed to just changing the oil.
- Mechanical shearing of the molecules. Engine oil (as opposed to transmission oil) is intended mostly for sliding in bearings. If you run it through gears, it'll actually end up getting cut apart (insert large handwaves here, search for mechanical shear stability if you want the full details) and thinning. Motorcycles that use engine oil in the transmission tend to be pretty bad about this, and on an old bike, I actually used this as the oil change indicator (the no oil pressure light would start lingering a hair longer on startup and that was my clue to change the oil). This, unfortunately, you really can't do much about without replacing the oil. Better quality oil tends to hold up longer, and different engines do this at rather radically different rates.
If you're rolling a ton of miles (especially with an engine that takes a lot of oil), various optimizations make sense. Bypass filters, oil monitoring, additive packages, etc.
If you're not? Just change the oil and filter every year if it needs it or not, and see if you can find "oil and filter" combo deals to save a bit on the oil. For most cases, there's no real reason to run a high end pure synthetic unless required, but I'd avoid the cheapest of the cheap as well. I tend to run a semi-synthetic diesel blend in most of the motors around here (truck, tractor, motorcycles, property motors - they've got more anti-wear metals in them than automotive oil for gas engines), and run whatever the car wants in it.
I have a lot of overdue oil changes after 2020, though. :/