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Ok, I am pretty good with engines, but this has me a bit stumped. I have a 1999 Kawasaki Mule (KAF620 4x4) that has about 2000 hours on it. I got it from my brother in-law,and I don't have a lot of history on it other than it had this issue for a while while he had it.

The engine starts right up, runs good, and seems to have plenty of power. Now the problem is that it burns oil (smokes) occasionally. But not in the predictable ways. It does not smoke when you first start it up (which usually indicates valve rod seals), but after about 10-15 minutes it will start to smoke like a fog machine. But then after a little while it will mostly stop smoking again. It will still smoke a little, but not bellowing it out like it does for a while. It will do this whether it is being ridden on flat ground or just sitting idling.

I have ruled out the radiator fan, as it does the same thing even when the fan is bypassed to run all of the time. I'm fairly certain the thermostat is opening properly as I don't get an over temp light and I have watched the coolant circulate in the radiator. I've pulled the plugs, and one time found the front one just slightly oily and the rear was fine. The next time the front was fine and the rear was slightly oily. There is no water in the oil and no oil in the water. Coolant level is fine. The oil has been changed and it does not seem to be burning any (i.e. the level is not changing noticeably). The crankcase breather is clear, and there is no oil in the air filter/intake. My BIL said he did a compression check (dry and wet) and it was within limits.

My only guess at this point is maybe a slightly blown head gasket that is opening/closing as the engine heats up. Anyone have any other ideas?

I happen to have a 610 but I have never had any issues with it so I have not paid to much attention to how the motor is built.  I do know a lot about engines.  Looking at the parts explosion of head gasket it appears that lube probably gets to the top end through the push rods and ultimately returns to the case by gravity drain.  The oil is separated from the cylinder by the water jacket so there doesn't seem to be a way that oil could get to the combustion chamber.   Is the smoke blue, black or white.  Does it smell like burning  cotton candy?   SO the oil level never drops at all?  If the color is more of a white or smells like cotton candy then it could be a head gasket leaking coolant.  Often you can run your finger around the inside of the tail pipe and see a trace of the coolant.

The smoke is blueish and has the typical oil smell.  The oil level is not dropping at any noticeable rate. I changed it 20 miles (not hours) ago and it seems to be at the same level. Not to say it's not dropping at all, just that it's not guzzling oil fast enough to notice yet. It always smokes a little after it's been running for a while. It's the fog machine thing it does for a short period that has me baffled. It bellows smoke for a few minutes and then stops. The engine does not sound any different when it does it. As noted, the coolant level is not dropping either, so I don't think it's a coolant leak. I've also pulled the muffler drain plug and it's bone dry.

there looks to be a crankcase vent hose between the front a rear jugs,  pull that off and see if full of oil.  Is there a baffle on either end of that vent that could have come loose?  You can also try to start it with that line off and see if you have much blow by.  That line could also be plugged/pinched causing to much case pressure to build.

That was one of the first things I checked. The line is clear and no oil. It "puffs" air as I would expect it to.

Now the breather valve is behind the alternator, but it looks like you have to pull the alternator off to get to the bottom screws of the cover.  But I'm not sure how that would cause the problem if the vent line is puffing.

if it does it every time on  a cold start I would take the air filter out just long enough to see if you still have the trouble.  With no oil in the breather tube and it is puffing, I don't see a reason to pull that breather cover.  You have electric fuel pump not an impulse pump correct? Want to rule out a problem with the diaphragm and  oil pumping into the fuel.

It does not do it every time (fog machine), but it happens fairly regular and only after it's been running for about 10+ minutes (plenty of time to have reached operating temperature, especially here where it's been in the high 90s). I have tried it with the filter off too. Basically I tried everything that might be causing oil to be sucked into the intake. And yes, it is an electric fuel pump. I have a gasket set, but I really hate to tear it down without knowing what is really wrong. And as noted, it pretty much runs fine except for the this.

I think you are about out of options other than to tear it down.The only other thing you could consider is to flush the motor in case you have gunky rings.   If the valve covers are easy to get at you might try removing them to see if there are any chunks blocking the passage of the oil return.  Since it doesn't do it all the time I would think it would need to be a lose chuck of deposits or  RTV.

I did a Sea Foam flush before I changed the oil. I have not removed the valve covers yet though. I may just leave it as is. It's not that big of a deal other than bugging me not knowing WHY it's doing it. I could tackle taking the heads off, but anything more and it would need to go to the shop since I don't have some of the special tools. I just have a feeling that if I pull the heads, I won't find anythig and it will still do it.

Hmmm.... Now I'm not so sure it's oil that is burning. To me it looks bluish, but my wife says it looks white/grey. And it smells like oil to me, but to her, she says it just stinks. And the other day when it bellowed a bunch, the coolant may have been a tad lower after that then before. One other thing that may indicate it's a head gasket leak (coolant) is that sometimes, the radiator will be under pressure after it's been running a while (as it should be) and other times it's not. Some googling shows that you can have an intermittent head gasket leak, although I've never seen one. I suppose just replacing the head gaskets would be the easiest thing to do. But I'm going to watch the coolant level more closely first, especially after it bellows.