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Oil consumption increasing as oil gets older

Looking back through my records, I tend to find the following (oil change interval, number of quarts burnt during the interval)

20,2
30,3
40,5
50,7

Is it really possible that older oil gets past the piston rings more easily?

I use 50/50 W80 and Shell 15W50, plus Camguard which has produced a big drop in the “hard” metals wear rate (according to oil analysis).

Administrator
Shoreham EGKA, United Kingdom

The older the oil the more unstable it will become, and more of it will be burned. It is typical that towards the change the consumption is highest and that right after the change it is low. It is my experience also.

My experience is the same.

LDZA LDVA, Croatia

The oil becomes “stickier” due to the collection of polymerized (growing in size, molecular weight, and viscosity) blow-by components. These larger, polar (having a stronger affinity for metal) compounds remain on the cylinder walls longer and are burned during combustion. I have mimicked this adding various thick baseoils of varying polarity. When adding 3% of a thicker baseooil, maintaining a constant viscosity, increasing the polarity increases oil consumption substantially. These same bad actors also go on to form the varnish deposits in the engine.

Ed

Interesting – thank you all

It does confirm my view that pushing all the way to 50hrs is not a great idea. I also found the metal figures in the oil analysis go up quite a bit past ~40hrs although when I posted that here in the past, some did not agree.

Administrator
Shoreham EGKA, United Kingdom

The rate of metals entering the oil increases substantially with time but it is blunted and convoluted by the addition of make-up oil. This rate increase is the reason I change my oil at 25-30 hours.

I have two Lycoming O-235’s working on a flying club and there is no detectable increase in oil consumption across the 50 hours between oil changes, while not disputing that fact that this might happen for all practical applications the increase is so small as to be insignificant.

The engine comes out of check with 5.5 USQ ( this is considered full ) the refill level is set at 4.5 USQ and this is usually reached at about 25 hours, the aircraft turns up for the 50 hour oil change with about 4.5 USQ in the engine, this has been the case for as long as I can remember.

Peter, I see the same and it’s quite drastic – basically zero until 20h then 1q every 2-3h. The amount of metal in my analysis seems to be pretty linear though. I change at 25h, very rarely more.

EGTF, LFTF

Mine starts to go up fast above about 35hrs.

Ed (above) is the developer of Camguard so he knows this subject…

Administrator
Shoreham EGKA, United Kingdom

denopa, I didn’t see that in my Mirage engine. Actually had a habit of dropping more at the start then slowing down as the engine seemed to prefer a slightly lower oil level.

EGTK Oxford
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