Doug is probably correct about the timer. Generally though, a timer is set at three to five minutes. You can, and should override this feature, by increasing the idle to 900-1000 RPM as Doug suggested.
Running at low idle is hard on these engines. Conversely, at high idle, the oil pressure should remain around 25-30 pounds. The engine will stay in it's operating temp range, and combustion will be sufficient to give you a good fuel burn. At low idle, the engine can cool down, then fuel will wash down the cylinders.
You will only burn about 1/2 gallon per hour at high idle. High idle won't hurt a thing.
For cold starts, I let em idle at low idle for about three minutes, then kick her up to high idle.
If you have normal oil pressure while driving, things are probably OK internally. How many miles are on your engine? When you have the limiter removed, have them download and print the complete history of your engine. This won't cost any extra.
If the history has not been previously cleared,
the ECM will tell you total hours on the engine, and mileage, which should agree pretty close to your odometer reading. The entire engine history is stored in the ECM down to the minute. If it's ever been overheated or over reved, the ECM will give you dates, times, how long. etc, etc, etc.