Coolant loss

In short none

 

You should have zero. Any loss of coolant means it’s leaking somewhere. Do an oil sample and make sure it’s not getting into your oil. If it’s getting in the oil, fix ASAP or it will ruin the motor. If it’s not in the oil and you’re adding a gallon over the course of 3 weeks or so, I wouldn’t worry about it as you probably have a leak that only appears when it’s hot and under pressure. If you’re adding a lot of coolant then you need to locate the leak somewhere.
If your truck has an APU connected to the motor coolant supply I’d start there. They may not leak when the APU is running or sitting still but once you’re hot and under pressure going down the highway it will leak through the hoses.

 

Next time you change the oil take a sample. I use Blackstone Labs and they do a great job. That eliminates a lot of things. Then you can focus on finding the leak. If you don’t see it on the ground during pretrip, it’s a high pressure leak. If nothing turns up from the sample or a shop searching for a high pressure leak, it could be an EGR cooler gone bad, that would be pretty rare though.

 

2 lb heavier cap should not be doing that.

 

The cap affects overall pressure in the system. It could lead to overheating but not any leakage. Since you’re adding coolant so often, you still have a leak somewhere. If it’s not in the oil it’s up to you if it’s bad enough to warrant pressurizing the system at a shop and having them check for leaks.
If there are truly no leaks, it’s going somewhere internally. An oil sample will tell you if that’s the case. You’re looking for Potassium and Sodium in the oil.

 

Well covered.