Some of the cheap oils out there are just that, others are actually quality product, possibly one series off current production of whomever actually refining it.
Little trick as to how to figure out what exactly is in those pails from Canadian Tire or the Wally mart...search for and check the MSDS for the brand name they selling the oil under...who ever packaged it will be listed. Then you can simply compare the specs and you know exactly what it is. For example, up until a few years ago, the Canadian Tire Diesel oil was Esso XD3 or whatever Mobil name it had when they switched, but it was two series off of what Esso was currently selling, but was still CJ4. I think now it is Rotella, but don't quote me on that. Haven't had to buy any for a while, lucked out a couple years ago when they stopped selling actual Esso/Mobil branded pails in the stores, I just happened to walk in as they were putting up the 40% off or such, ended up around $32 a pail and needless to say I took every dam one they had in Athabasca. Took two trips to get it all home, don't get those kind of deals often lol.
Now they sell Shell branded pails besides their own, but not as much diff in price as it used to be years back.
As for air filters, I leave mine alone as much as possible. Depends on machine as to how often I will check them, most have some kind of monitoring system, best of all is when there is an actual vacuum gauge or one of those mechanical restriction type indicators. I blow mine out with air, but one needs to becareful you don't damage them. Always a good idea to check with a light for breaks or tears in the paper afterwards. Never even think of cleaning the safety elements, and those never get changed unless determined to be necessary.
I am a lot more fussy about air filters than oil and I stick to OEM or high end brands like Donaldson. Compare the weight and number of pleats in many of the other aftermarket ones, there is no comparison, you get what you pay for IMO. I have equipment I have run thousands of hours with the same filter with only occasional cleaning, it is highly dependent on how good the aspirator and more works, location of air intake and work being done that plays into it as well.