Spark knock issue

My car suddenly went from 240 miles per tank to 150. Performance does not seem noticeably affected and not burning rich (no black smoke or raw fuel smell), but I noticed that when taking off from a stop there is a noticeable spark knock rattle and same upon heavy acceleration that was never there before. I installed mechanical temperature gauge to get accurate temp reading and noticed the spark knock was gone although the power suffered due to PCM not getting temp signal. Have had excessive oil on top of valve covers. EGR problem?