Well that isn't really true... draining the cooler replaces 100% of the oil vs approx 85%... but it shouldn't make that much of a difference if you change it reguarly.

I didn't do my cooler install but mine is facing down. I've always heard this wasn't the preferred orientation because it could hold air. Mine does seem to working well however.

I'm not convinced of the backflow theory since the oil gets sucked from the pan, into the oil pump, through the oil filter, and then through the oil cooler and then through the oil passages in the engine. Most filters have anti drain back valves built into them so it probably couldn't go that way. So that leaves the passages, which go throughout the motor that it could, backflow through. I guess if any of those passages are below the oil cooler it could backflow through them.

I wonder if the difference in what modshack saw and what the spearfish saw was the difference between having a filter with a good anti drain back and one without.

I have my oil cooler fittings facing up and the last few oil changes I did not drain the oil cooler + lines. Blackstone results don't show any noticeable differences so I'm ok with that.

Note: in the past I would unmount the oil cooler and invert it so that the oil would flow to the sandwich plate. This worked pretty well, as opposed to the one time that I removed a fitting and almost had a leak. Never again.

Yeah that's my 2 cents.

ahh interesting. reason i asked is because of this actually...my last few blackstone UOA's said to up the intervals to 5k. but if im going to be basically only changing 85% of the old oil out to put in new oil, i should think about bringing it down?

Whatever makes you feel comfortable.

The most I've gone before an oil change with this Z is 4500 miles. The oil was still good and could have done 5000+ miles according to blackstone.

i have a 28 row oil cooler with the fittings facing up. id like to think that i am running enough oil but now you got me second guessing. with the engine off it reads high. but, i refill the cooler (3/4 of a q) first during an oil change. then i do the standard 5q of oil the engine requires and then give it a reading after letting it run and then settle. like i said i get a high(ish) reading. please tell me i’ve been doing this correct and not breaking my engine lol.