Suggested oil change interval

I have a "If if were your Corolla" question.

Mine is a '98 LE with ~73K miles; starts and runs pretty much like a new one; does not consume oil. A couple of oil changes back I started using full synthetic, currently running Pennzoil 5W-30 Platinum High Mileage--next change I may try something different.

My question: If it were your car, how many miles would you put on this oil between changes? (I'm less interested in your opinion of Pennzoil)

Personally, I put literally about 5-6K miles on this car in a year.... maybe 7K.

I'm sorry nobody really answer this for so long. But its a good question and I happen to have an answer.
First, Keep using synthetic, the uniform oil molecules are a key to maintaining reliability and preventing wear.
The Weight of the oil is based on climate and bearing clearance. The owners manual shows the temperature vs oil weight for your climate but in general the 5W-30 is most commonly ideal.

Next, there is a company called blackstone oil

If you can take a good sample of used engine oil and send it there, they will give you an idea of what oil change condition and frequency.

In my opinion, I will change oil no more than 3,500 miles in any daily driver vehicle with a turbocharger. For performance engines it could be as soon as 2,500 miles when the engine is very old (20 years or so). The corolla of course does not have a Turbo so the oil does not get beat up as much, but still due to it's age and the fact we wish to maintain it as best as possible, I won't put much more than 3,000 miles on the oil before changing.

Oil does more than lubricate, it also captures and carries debris, carbon conglomerates which form deposits and sludge. The PCV system works with the oil to keep deposits from forming and prevent the dark goo sludge from forming hard deposits around the engine. Therefore, it is imperative that you MAINTAIN the PCV system while you change the oil. You Must inspect the pcv hoses, clean under the oil cap, replace the pcv valve as needed, and make sure there are no leaks in the crankcase such as valve cover gasket or oil dipstick seal. You should keep using the OEM air intake system with the OEM pcv attachment and OEM paper air filter. These things, the intake system, the air filter, the pcv system, the seals and gaskets, all work together WITH the oil system to maintain the quality of engine oil and enable high mileage and reliability.
Actually the longer time duration the vehicle sits for a given mileage the more often the oil needs to change, especially if vehicle is used for short trips.

Oil pulls water from air and especially while cold on a freshly started engine for the first 30-40 minutes most cars.
It takes 30-40 minutes for an engine to warm oil to roughly 212*F boiling point of water afterstart.

During running condition, crankcase fills with water. Primary byproduct of combustion is water and CO2. So the blow-by gas is filling the crankcase with water vapor while the engine is running. This water interacts with engine oil and enters the oil due to partial pressure. If the oil does not warm to at least boiling point of water for long enough period the oil will hold the water and when the engine is shutdown the water will interact with the oil and engine internals.

Water in the atmosphere also constantly filters into engine oil through open crankcase vent while the engine is off.

As the oil gradually accumulates water the amount of water which is leaving the oil increases the next time oil reaches near boiling point.
The water vaporization mass will increase each time the engine accumulated water before shutdown and the next cycle the engine oil is heated remotely close to boiling point for water the water vapor will ascend to the valve cover (higher points of the engine) area and begin to accumulate sludge into the oil orifices and oil cap area of the top of the engine.

A good test to see if this is happening is to remove the oil cap and inspect under the oil cap for accumulated sludge. This would be an indication there is too much water in the oil, driving durations are too low and oil change intervals are too long.

et al;
It isn't always about mileage on the oil. It is also important to consider water accumulation, CO2 -> Carbonic acid (CO2 in the oil in presence of water), and the influence of warming oil on oil reservoir.
1998 Corolla is very old. This is from the manual

Original oil change interval is around 5,000 miles. But there are special considerations for vehicles operating at low speeds extended periods or short trips for the original brand new vehicle. Additionally because these cars are now 25 years old and have had many previous owners and probably over 120,000 miles there is the issue of previous mal-maintenance and the effects/influence of high mileage which reflects oil change interval. That is, originally with 0 miles on the engine for the first 10-15 years or so up to approx 120,000 miles with normal driving and usage the original oil change interval of 5,000 miles is acceptable. However once they reach 15+ years and 100,000 - 120,000 miles range the crankcase needs to be kept cleaner and more frequent oil changes are needed, 3,000 miles is more ideal. Oil acts as a cleaning agent and the PCV system supplements the cleaning action and over time many previous owners have not properly maintained the proper specification of either and due to normal wear and occlusion of environmental and combustion products the more frequent oil changes at high mileages are needed.
Just happened to see this, about oil change frequency, always change it sooner than the manual recommends,
this is what I am getting at as with mileage on oil, the higher mileage the engine, requires sooner oil change frequency

12:55 look at the oil control rings, seized in the ring lands, caused excess wear

The ring seized because of carbon deposits, which could have been prevented with more frequent oil changes and a superior PCV maintenance regimen.
Brand new engine 4k to 5k is 'fine', but after 100k miles there is a gradual building up of carbon reservoir circulating, you want 3k oil changes or even less depending how the vehicle is used.