</font><font color="blue" class="small">( Bird, it is a 258, what do you think of that model, how does it compare to the 261? I paid $300 and I know I could have got one on line cheaper, but I needed it right away. I notice that it has an oil resovoir, that strikes me as unusual, is that a standard IR design feature? Any problems with that? )</font>
I don't recall ever working on a 258 so I'm not sure how long IR has been making them. However, I just looked up the exploded view and parts list for that tool, and it's a very common design, used more by Chicago Pneumatic (and Snap-On), as well as a number of cheaper and weaker knock-offs including Central Pneumatic, than by IR. There's certainly no problem with it. Just as a lot of our members prefer different brands of tractors, mechanics prefer different brands of impact wrenches. It's an entirely different design from the 261. The 261 has a hammer frame and one large hammer, and has a grease fitting in the back end. The grease is forced through the rotor to lubricate the impact assembly in the front end. The 258 has a hammer cage (big, heavy steel cup affair), 2 hammer pins, cam, cam release spring, etc. that are lubricated with SAE 20W or 30W non-detergent motor oil. That design just has a lot more moving parts, plus some oil seals to keep that motor oil from leaking out, but it works just as well.
I assume you got a manual with it that includes the exploded view and parts list, as well as maintenance recommendations. It should have come with the right amount of oil in the front end and may not need any added for years. If you do need to add, or check, the oil level, just remove that plug, set it down on its top, and turn it so the oil plug is at the 3 o'clock or 9 o'clock position as viewed from the front or rear end. In other words, you want that front end about halfway filled with the motor oil. And of course, as with any of them, you apply air tool oil to the air inlet.