I'm not sure if I'm being didactic, but may as well be straightforward. I'm also not an expert on oil, maybe Chris knows more on this topic. Oil is more viscous that water, so it should be easier for water level variations to reveal anomalous pressure pulses than oil. Water is also more closely tied to fauls than is oil. Several deep wells have found fault zones to be filled with water, I'm not aware of any well that struck high-pressure oil at a fault. So water wells and strainmeters are more sensitive ways to watch for anomalous behavior, and ones that have been watched for a long time.

That said, we are interested in looking for strain transients or pressure transients associated with earthquakes. The plot you connected to is not obviously significant, however. Many such strainmeters have large signals unrelated to ground deformation, in fact one proposal has been to insert strain meters in trios, as any one is not to be trusted. Sometimes it takes a while for a site to "cure", some sites never lose the large signals that we know to be noise because they do not appear on adjacent instruments.