why do I have to add oil between oil changes

Consumption. I believe I said that correctly.

The new oils are much cleaner and thinner. They burned off without notice from the tail pipe.

Try a oil with higher specs. What are you using in it now?

Assuming there are no gasket leaks or loosing it to the cooling system.

May check your oil filter gasket or drain plug. Look under it for signs of leaks.

Mobile one is better. If your using a 0w-? or 5w-? oil go higher. Where is the general area you live. Like northern or southern climate? What is the coldest and what is the warmest weather there. How many Miles on it? Is this the north star engine? If so and even if other these year cads have intake casting to narrow and the gaskets fail repeatedly along with the head gaskets which have issues so there is not much can be done unless you glass the engine AKA Sodium Silicate Na-40 properly. The gaskets can let coolant into oil or exhaust or vise-versa or out of the motor all together.

Try lubegard oil treatment in the oil and maybe the lubegard coolant treatment for the coolling system. If your having normal too thin oil burning issues then there is no problems and just use thicker oils.

The 3.6 v6 that came in the Base SRX is a nice engine but in 2004 it was the beginning of the production run. There are problems with the oil seals and some engines burn a lot of oil straight from the factory. There were a lot of complaints early on with the 3.6 and many of the engines were replaced for oil consumption and / or timing chain failure. DO NOT let the 3.6 run low on oil, even a little bit. The chains do not fail in a catastrophic way but they stretch and throw the timing off a little bit. These problems were fixed in 2009 or so and the V6 3.6 has become a great engine but not in the early SRX.