The stock 7.4's I had apparently held 7 qts of oil however I only remember putting 6 at every oil change and that includes me filling the oil filter before install. Showed full so now I am thinking a qt or so was in the cooler and lines? Anyway, I now have oversized coolers with more oil lines and I am pretty sure that it took almost 9 qts the first time to fill to capacity. I did have an oil leak during the whole process but I still think 9qts is pretty close. FYI, I am using the same oil pans. I ended up mounting the new coolers to a center stringer below the oil pans and it just occurred to me that there may be up to 3 qts of old oil left in each system at every oil change. That seems like too much and I need to figure out how to clear the system each time. Was thinking of disconnecting a hose and putting air pressure to it? I can install a quick disconnect in one of the oil lines that is permanent. I don't know if there are valves or whatever in the block that would keep me from essentially back flowing oil? Plus I'm pretty sure you can't force oil back through the oil pump? Also wouldn't want to back flow the oil filter itself. Looking for ideas before I try to "reinvent the wheel." I already have an oil evac system hooked up to the drain of the oil pan. Maybe a simple "T" in line with the drain line from the oil pan? Thanks for your help.

The stock 7.4 is 8qts when 100% empty. If you suck the oil out through the dipstick, usually it will only take 7qts to fill it.

No way your lines and cooler are holding 3qts. Prob around a quart.

On another thread I am replacing the harmonic balancers and seals and when I removed the balancer I smelled burnt oil. I've had the valve covers off a few times and did not smell anything unusual. For my piece of mind, I would really like to drain 100% of the oil if possible.
I measured the total length of hose I'm now using to run from the engine through the filter to the thermostat through the cooler and back to the engine and there is a full qt of oil there. Also the new oil coolers I bought hold about a qt. There's another qt in the filter but that gets changed out of course. Total system capacity is 9 qts.
I am using the stock remote oil filter assembly. Can I remove one of these plugs, apply air pressure and blow the 2qts of oil back into the pan for removal or is there some kind of check valve in the block to prevent this?
Thanks again for your help.

Imho - i wouldn’t care about several quarts not being changed if:
-you change the oil here and there like normal perf boaters (once a year or so)
- you have not had a catastrophic engine event/water got in in oil/etc/etc

No big deal.
-

Probably easier to make several oil changes in a row, running the engines for a few minutes in between, if you really want to make sure that you replace all the oil.

I wouldn't be too fuzzed, though.

To answer your question...yes.

I'm sure your engine builder appreciates how worried you are about clean oil

I usually do. Thanks

Lol!! I'm sure he does.
Quite frankly, It wasn't until I made these engine upgrades that I realized I wasn't changing ALL of the oil. It bothered me for that very same reason that sucking oil through the dipstick tube was not doing the best job of removing ALL of the oil so I installed a system where I now pump it out through the drain plug location. Maybe some of you still suck the oil out through the dipstick tube however it bothered someone else too and they came up with a solution and I bought it. People who get you is priceless.
Isn't there some irony in how harsh the marine environment is and the stress on marine engines and how we carefully monitor the oil temp and pressure and then we don't change all of it?
It doesn't matter to me that I'm the only one(?) that is bothered by this but it does and I can't really help who I am. We all have our little demons don't we.
Lol!! Thanks to bajaman for answering the original thread question however there's another question buried in the post and I am wondering if anyone can answer it.
Can I apply air pressure to one of the other inlets on the remote oil filter location and force the 2 qts of oil back into the pan for removal?

I really appreciate your time and comments.

There is no need to go through all that trouble, Rob.

If the oil is going through the filter, there's no water in the oil, and no failures, then there is no reason to be that nitpicky. Seriously, I'd just leave it.

One thing you can and should do is occasionally cut open the filter to see what's buried down in the pleats. You can get a decent tool from Amazon or Summit or Jegs (don't use a hacksaw or cutoff tool to open - it'll contaminate the media). Make sure to get some rubber gloves, too (it's a messy job).