When you drained the oil...what did it smell like?

Having a small % of the remaining old oil will not be an issue unless it has particulate contaminants etc.

Fuel dilution is one of the primary reasons to change engine oil. Mostly from blown applications with aggressive F/A mixtures.

If you're concerned about your oil...and when to change it...do a few oil analysis inspections.

Trying to force oil back through the oil lines towards the oil pump, it's not going to flow past the gear pump very well, it would have to bypass on your bypass system if you have one. That's a pretty steep pressure to just get a small volume of oil out of the lines.
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The biggest culprit would be the oil coolers if you got larger ones.

What evidence did you determine/find that all the old oil needs to be removed?

The easiest thing to do is just change the oil before it needs it. Have some good filters on. Maybe a System 1 re-usable filter.

When I do oil changes on my Dodge Cummins engines, I always have clean oil for a long time after the change, despite the sooty diesel oil that came out and left in etc. Good filters really help here.

Using oil that is heavier, like a 20w50 is good protection from fuel dilution and shearing destruction.

Dude I agree and also am very retentive about oil change, but you will never get 110 percent oil out of motor, every lifter, oil gallery, every low spot in cylinder heads lifter valley etc.the best way is to run the engine up to temp drain it, fill with fresh oil run it up to temp drain it and fill with fresh oil. I go get the cheapest synthetic oil and do it twice, the engine cares less about the brand of oil versus how often you change it. It may seem wasteful but I drop off oil at depot. All my engines have oil so clean it’s hard to see it on dipstick, you say it only takes 6 quarts at a time, well for flushing purposes just buy 3-5 quart jugs at wall mart with some filters and flush it 2 or 3 times maybe costs about 100 bucks.maybe throw in a bit of seafoam each time

Guys,

Whenever I'd changed my oil, which I only started doing myself when we got the PQ, it always bothered me that I only got about seven quarts or so out of it, and it only took about eight to refill it. I knew we were losing some to the effects of the ProCharger and the way the 496's PCV valve and oil dipstick worked, but the short fill, compared to the spec'd capacity always bothered me, because I knew I wasn't getting it all out of the pan. When I did the oil temp gauge install, it included an oil change in the process, having to remove the remote oil filter head during the process. This time, however, I used the OEM drain on the side of the oil pan, with a hose out the drain plug hole in the transom. What I found was that I got almost nine quarts out of it and that it took a full nine quarts to fill it when the job was done. Quite the difference.

Takeaway....? If your engine has such an oil pan drain feature, use it. You still won't likely get the oil lines cleared, or any of those pesky residual retention spots, but you'll certainly drain the pan and hopefully take some of the debris that starts to build up over time with it.

Thanks. Brad.

I don't want to force it back through the pump. I want to "force" it in the normal direction of flow from the filter, through the cooler back to the engine. I have been going through this thought and now am quite sure it won't work because all that will happen is I would probably be able to clear through the thermostat but the cooler itself would remain full of oil because of the bypass in the thermostat. So this effort would not accomplish my goal.

I don't really have evidence that all the oil needs to be removed. I think everyone would at least agree that removing all of the oil each time is better (however you would measure that) than replacing some of the oil. In my case I might be changing only 60%.
I normally change the oil at the end of the season. For the past 2 seasons I only put about 25 hrs on them each year. I think some of you guys change your oil at 20-25 hrs so I'm "accidentally" right there with you.
I run the Wix filters (not race) which I believe are probably the best ones out there. Everyone has an opinion. Those reusable filters don't filter down to the same size as the Wix filter does it? My brother had one and I don't remember the exact spec's but Wix filtered down to a smaller number.
I don't think running a thicker oil with my engines is a good idea. My engine builder told me to run the Brad Penn 10w30 Semi synthetic so that is what I'm keeping with. I actually buy it from him.
So I'm not going to pursue this idea after all. I have come to the realization that the "Juice is not worth the squeeze".
Again I want to thank everyone for their input.

I saw an episode of engine masters where they tested all different oil filters and flow rates, I believe the k&n gold filtered down to 10 microns and had same flow as wix and all the others that were 20 microns, also I wasn’t necessarily saying to switch to full synthetic but just to change oil twice in a row, oils cheap and if you do it twice then if you say you are only changing 60% then do it twice and then left over oil will be diluted with more fresh oil,

You can watch engine masters on utube, I only do the twice oil change for the winter layup and come spring time I have fresh oil( have heater in engine compartment cause I love my big block) and my wife pays power bill

I may do the twice oil change next year.
I really enjoyed watching engine masters. Sad that Brule retired and then motortrend literally dropped every show I liked so I dropped them.
I keep my boat in the basement because I love my entire boat

Omg am I jealous, I have an old 1989 2755 Bayliner Ciera it has a radar arch and is about 10 or 12 feet when on trailer, it’s my sleeper boat, came with a skanky ford 351 and Omc drive, now it has my tall deck 496 aluminum heads roller cam aluminum water manifolds, aluminum water pump and mini starter, probably lighter than the all cast iron 351. Also converted it over to my Volvo 290 dpe drive and thru hull exhaust.