I was reading Clancy story about his trip and he said he didn't had to add extra oil although he was riding in hot weather and twisty road. If it was my bike I sure would had added quiet a few ml of oils.

Some GS don't consume oil but most does like mine (yet she is only above 40k Km) even though I ride very soft. Does any one have an explanation for this?

If your GS is consuming oil does it mean she will last less than a GS that don't?

4 stroke engine designs like the GS are not supposed to burn or consume oil as part of their normal operation when everything is upto spec. That being said, burning almost a quart oil over a few thousand miles is generally considered acceptable for some used/higher mileage engines. It doesn't mean the engine is about to fail for sure.. but it is the beginning of the end (or at least a rebuild).

My 02 has 10,000 miles. I've replaced the oil at 3k, 6k, and 9k miles and have never had to add a drop in between oil changes. Some members have claimed that air cooled bikes consume more oil in hot weather, which kind of makes sense.

Not really an answer to your question but Toyota does not consider oil consumption "excessive" for warranty porpuses until an engine starts to burn a quart every 1,200 miles.

-Jessie

Rotary engines such as Mazda RX-7's use a lot of oil.

All internal combustion engines consume "some" oil.  IMO.

It is just not possible that every molecule of oil is squeegeed to where it belongs by the rings; every piston stroke. 

It is the oil that creates the 'seal' between rings and cylinder wall... so every ignition equals 'some' burnt oil.

Also valves are sealed by oil (oil between valve stem and guide/seal).

Tolerances not being correct (either by assembly or wear and tear) will certainly accelerate oil loss.

Why an air cooled engine would use more oil when in hot weather... no idea.  I would have thought the extra heat would improve piston ring seal....  Unless the engine is getting too hot... then all bets are off, and we're just lucky the engine isn't locking up.

I have owned several engines (cars) that burned a quart every 2k miles... if i wasn't really paying attention; most i would not have known about it.  Only the other issues with the engines lead me to check oil level, and discover it was burning oil.  Others it was quite obvious by the smell of the exhaust.  Two of them i 'rebuilt' (mustang 302, honda 1., and found that the combustion rings were okay (ish)... the oil rings however were 'burnt' and brittle, no longer springy.... and the valve seals were completely shot.  So, in my limited experience it is overheating that caused my oil loss.  (both of those vehicles had overheated the engine more than once... due to water pump failure or radiator/fan failures).  A third engine i had (vw 2.0) had bent 3 valves, oil burning was obvious, and basically not drivable; but i did not rebuild that one (found bent valves and junked the engine).

@ temperature when oil gets splashed on the hot parts, obviously it evaporates. That would account for why the oil gets thicker as it ages, at the same time it loses its lubricity. Anyway my 89 used to thicken the oil and you could actually smell it, off the motor, not out the pipe.
Cool.
Buddha.

I agree with the others, but I want to add that once a 4-stroke engine accumulates many thousands of miles the first thing that normally happens is oil begins to get past the valve stem seals.  It'll smoke when you start it cold, particularly on cold days.  Seconds later the smoke stops.  If the seals are only leaking a little you won't see smoke.  Mine does that.  That is "normally" where most of the lost oil is going until wear becomes excessive.

I've seen no indication of any excessive valve stem seal wear on my GS500s in the over 80k miles I put on my 97 GS or on my 02 GS that has 84k miles now and counting.  JP had a 82 GS1100E with about 50k miles on it he rode with me on my 97 GS back in 01 to the Honda Hoot in Knoxville and over the Dragon and back up the BRP to I-77.  We stopped for dinner one evening and when we came out an hour later and he restarted his bike it smoked like a chimney for a minute and then cleared up and stopped smoking.  That's valve seal wear, oil leaks down past the seals into the combustion chamber when hot and is burned up smoking like that unless it sits overnight and the oil leaks on down past the rings to the sump and then it leaves no excess oil in the combustion chamber to burn and smoke.

The GS is an air cooled bike so the pistons are fit looser than on a water cooled engine to avoid seizure if the engine is seriously overheated.  You might say it's half worn out when it's new.    As the miles build up and the rings wear the amount of oil sucked into the combustion chamber past the rings on the intake stroke gradually increases and oil consumption increases with it.

Both of my GSs went the same way, they used very little oil for the first 30k miles or so and went the 2-3K miles between my changes without adding any oil but with the oil level getting closer to the add mark as the miles progressed.  After that the bikes required more and more oil as wear progressed and riding them at high rpm's when hot they would use even more oil.  With 80k miles on a GS and riding it moderately I still get one gallon oil changes, I put 3 quarts in to change it and use the other quart to top it up and change the oil when that gallon is used which is still 2-2.5K miles between changes but the consumption can be increased a lot by giving it a good hot thrashing.  

Quote from: The Buddha on February 28, 2011, 09:33:26 AM
@ temperature when oil gets splashed on the hot parts, obviously it evaporates. That would account for why the oil gets thicker as it ages, at the same time it loses its lubricity. Anyway my 89 used to thicken the oil and you could actually smell it, off the motor, not out the pipe.
Cool.
Buddha.

That is not entirely true for most production engines (4 stroke) or oils. The vaporization temperature of (most) motor oil is over 400 deg F (Shell Rotella T6 in my case). A decently sealing top piston ring in a petrol engine would be just about the hottest part in a motor that the oil would touch. Even the upper extreme of temperatures of this top ring (the portion of it where the oil hits) reaches only around 300-320 deg F which is not hot enough to vaporize the oil. Also, multi grade oil gets thicker at higher temperatures because of added polymers that sorta coil up to increase the oil viscosity as oil temperature rises. Old/"dirty" oil feels thicker or sometimes even flows thicker because of carbon/soot/very fine metal particles/other contaminants that accumulate in the oil over time.... not because some of the oil boils off leaving a thicker oil behind

Quote from: The Buddha on February 28, 2011, 09:33:26 AM
@ temperature when oil gets splashed on the hot parts, obviously it evaporates. That would account for why the oil gets thicker as it ages, at the same time it loses its lubricity. Anyway my 89 used to thicken the oil and you could actually smell it, off the motor, not out the pipe.
Cool.
Buddha.

This this this. Different oils have different volatility, meaning that they evaporate at different rates. Group IV synthetics are among the best this way - I found that running Amsoil in my bike really helped keep the burn rate down.

Air/oil cooled engines tend to consume more oil than water cooled engines. This is partially due to looser tolerances, and partially due to the higher temperatures that they run.