I have my own vehicle in the shop today, a 2008 Polaris Sportsman ATV. My wife was driving it a few weeks ago and she called me and said "something exploded, there is oil everywhere!" This first photo shows what I found when I went to get her. The oil reservoir had actually exploded.
These Polaris ATVs have a 4 stroke engine with a dry sump oil system. Unlike a typical engine with an oil pan or sump on the bottom of the engine, these engine have a separate reservoir to hold the oil. This allows the engine to be more compact and probably makes it easier to increase the ground clearance under the engine. I think that it also helps with cooling the engine, the oil reservoir has a large surface area to dissipate the heat in the oil. (There are probably more reasons than this to use a dry sump, but these seem like the obvious ones.)
I have seen this problem several times before on other peoples machines. ATVs in the village make a lot of short trips in cold weather. They do not get run long enough or hard enough for the oil to heat up. This allows moisture to build up in the oil and the vent line on the reservoir gets plugged with frost and ice. Once the vent is plugged the pressure builds up and blows the side of the tank off. Polaris is aware of the problem and has tried to prevent this by putting a small slit in the vent line. This slit in the rubber hose is supposed to stay closed during normal operation and open up if pressure builds in the line. Unfortunately it seems like the blockage happens right at the tank and the pressure can not be relieved.
Another potential problem with the moisture build up is the possibility of ice forming in the bottom of the tank when the vehicle is parked in the cold. If enough ice forms it can block off the oil supply line to the engine. This of coarse leads to an oil starved engine and all kinds of problems if the operator is not aware of the situation.
This photo shows the new tank and the original. It is interesting how the whole side of the tank exploded. This is the same way that I have seen other ones fail. I am surprised that the tank doesn't simply split open?
To try and avoid future problems I have decided to insulate the tank to help the oil heat up better in cold weather.
I cut up an old camping type sleep pad to cover the sides. I am using DOW 732 silicone to glue the closed cell foam to the tank. 732 is great stuff, I use it for everything.
Here is the completed tank ready to install.