Well I have an update. My friend filled me in on the details. The motor/pump is newly rebuilt (about 50 service hours ago). Supposedly mud daubers got to the motor during a period of time where it wasn't in use and not all of the debris came out after the discovery. But it ran for about 3 years this way before needing rebuilt. He seemed pretty knowledgeable with it, and hooked it all up (including a new electrical setup) without a hitch. I cringed a bit at the completely dry and caked K&N-style air filter that he said to "blow out" once in a while (that will be gone asap, he gave me permission to change it). But the oil? He said to literally use any 30 "weight" (I hate that word, it's viscosity...) I want, as long as I keep the sight half full of oil. 0W-30, 5W-30, 10W-30, SAE30, ND30... Does not matter. So especially considering the filter, I'll change the 'mystery' garbage oil with probably a 5W-30 or 10W-30 to help it out with the couple of inevitable cold (+5°
Since some of you pointed out that it is technically the incorrect oil and will keep contaminants suspended, I'll just change it per the manufacturer's overkill schedule. After I change it, I'll keep my eyes peeled for ND30 on the cheap.