Hard soil is usually caused by the type of clay and a lack of moisture. In the Willamette Valley, it is common for the soil to get really hard when it dries and it will also sometimes develop big cracks. The way to prevent this is to irrigate consistently.
It is always a good practice to aerify in the spring and fall when the ground is neither too hard or too soft (wet). The main benefit of aerification is to improve water infiltration and remove compaction caused by heavy traffic - usually vehicles. Do not aerify now as the holes will not heal in time for summer.
Lime is applied to raise pH which can be beneficial but takes a few years for it to change the pH and will likely not make a big difference in the quality of your lawn because most of the pH's in the Willamette Valley are adequate for growing good turf (i.e. in the low to mid 6's.) Unless, you know that your soils have a low pH (i.e. in the 5's), I would not apply lime.
Gypsum is usually applied to remedy high sodium soils which is not a problem in the Willamette Valley.
You did not say anything about your lawn. How does your lawn look?"
If you irrigate it regularly, fertilize it when it needs it, and mow it often it will look good. Aerify it this fall and again in the spring if you have water infiltration problems or compaction.