No recos but I'd run a 10W40, 20W50 (if you can find it) or 15w40 HDEO for now. The latter is readily available and inexpensive and actually really good oil. Delvac is probably still ~$14/gal at WM

 

thanks for the reply. I dropped Castrol 10w-30 syn blend and the noise is all but gone. Going to take it to a shop today to see if they can confirm that it's a rear main bearing knocking. Still more evident in the bell housing than engine so maybe the noise is traveling to the most hollow spot closest to it... i dunno. Seen big rigs do that though.