A friend of mine has a 8V92 Detroit in a boat, it uses between 1 - 1.5 gallons
of oil a day, Delo 100. Is it necessary to change oil when the consumption
is that high? Mathematically he changes oil every 5 days.
Spin on a new filter and call it good?

 

WOW, that sounds more like a leak. Years ago when the silver 92's came out I had the chance to be part of a situation like you describe on a friends KW, it turned out to be the blower seals leaking pretty bad and when he shut it off, no outside leaks but it's leak internally and during running it just burned the oil without any major telltale signs.
It sounds like you have a leak, not consumption problem.
Just my 2c worth.

 

I agree with Kenny. Check this first.

 

all of them leak, even before they put oil in them at the factory. you are correct you will never need to change oil and the filter can go longer. and as cdennyb stated some of the leak could be internal. I have the worst luck with the 71 and 92 series, got plenty of experience working on them. down to two of them in my fleet, one a 2 -71 in a Lincoln welder the other in a peterbuilt. if he works on it be sure to help him. you will never see another engine built like it. 4 exhaust valves per cylinder. no intake valves, big holes all the way around the cylinder located about half way down the cylinder and is two cycle.

 

I think bad blower or turbo seals would show up at the air box vent tubes? Make sure the vent tubes aren't plugged so they can drip like they are supposed to. It's been so long since I've been around 2 stroke DDs, I can't remember if plugged tubes will increase oil consumption. Craig

 

Try running a single viscosity oil like 30w or 40w instead of multi-vis.
It will probably at least slow the usage down.

 

Way back in the early 70's I worked on transit coaches for SCRTD and we had some of the early fleet from the late 50's and 60's and they had 6v71 2 stroke Detroit's that had well in excess of 2 million miles on them and they would take 1-2 gallons on Delo per rollout, the backs of the coaches were always slobbered with oil and road grime.

Make sure the airbox drains are open if it has them, leaking blower seals should become pretty evident when they get bad enough.
make sure your Air Damper (emergency stop) is functioning properly incase you experience a runaway.

You can do a leakdown test.

 

Grew up around Detroits and learned that "If it ain't leaking.....something's wrong!!" Well, that's not the case. Sometimes the piston assemblies were done correctly....no leakage.....sometimes they were done incorrectly....LEAKS. The incorrect piston assemblies let compression get into the crankcase via the wristpin caps and blow the oil out of the seals. They were (and still are) engineering marvels in IMHO.....they were way ahead of their time!

As stated above....the leak could be internal and just burning while the engine is running. If it's slobbering everywhere then it's just a matter of tracking down the leak point and fixing it. Being as it's in a boat.......EASIER SAID THEN DONE!!!