Only 10w cat oil ? We have a exxon dealer with 10w
Apart from some specific exceptions (and your 315C is not one of them) Cat hydraulic systems generally specify Cat Hydo Advanced SAE10W oil or a "commerical SAE10W hydraulic oil".
Sometimes the change period will be cut, e.g. 4000 hours using Cat HYDO oil, 2000 hours using a commercial oil.
You can use any good-quality branded SAE10W (or ISO32 AW) hydraulic oil. IMO Exxon would fall into that category.
Tip: If the supplier has an SAE 10W oil that meets Cat TO-4 specification
and if it is cheaper than their 10W hydraulic oil, buy that. Your machine hydraulic system does not require the TO-4 oil, but neither will the TO-4 additive package do it any harm. We used TO-4 oil in all our small/medium Cat equipment hydraulic systems.