Is it mandatory to change oil once a year?

It's 15,000km or 1 year, which ever comes first. If you only go 4,000km in 1 year, then you should change the oil at 1 year.

To add to what @BrianKnoblauch said, there are other reasons why I wouldn't let it run that long. The main reason is the additive package which is delivered in the oil breaks down over time. Once the additives breakdown, the oil will no longer clean like it's supposed to, nor have the viscosity variance we see in multi-weight oils. Once this happens, your engine will start seeing greater wear, which over time adds up.

EDIT: Something I did fail to mention is if your vehicle is under warranty, you will want to absolutely follow the manufacturer's guidelines if you expect to maintain the warranty. If you don't care about it, it becomes much less of an issue (I do not see most people letting the manufacturer off like that, though).

Full disclosure: I'm no expert on oils, this is just some of the information I've gathered from researching on my own.

To add to what @Paulster2 and @BrianKnoblauch said, one of the reasons a time and mileage are both specified has to do with the what happens as the oil is used and as it ages. Part of the oil's job is to neutralize the by-products of combustion. As the oil is used, these by-products start to contaminate the oil. The more contaminated the oil is, the higher the acidity becomes and the more potential for corrosion to your engine. Another thing that happens at the oil ages is oxidation, which again raises the acidity of the oil.

To get an idea of how much life is left in the oil, you can take a sample of the oil when you change it and have it analyzed. Blackston Labs is a popular choice.

The analysis can tell you, among other things, what the viscosity is currently at (it will usually be a little lower than the virgin oil) and it can tell you the total base number, which relates to the oil's acidity and ability to neutralize by-products. There are other test that give you an idea how much of the "cleansing" additives are left, and all of this together will give you a very good idea of how much life is left in the oil. Consequently, this information will tell you if you could have gone longer or should have changed it sooner.

If you do this with each oil change, you can easily come up with a maintenance schedule for your particular vehicle/conditions/oil choice.

Another benefit is if you do regular oil analyses you can often spot engine wear before it becomes a catastrophic failure. At $20-30/oil change, it's pretty cheap insurance to invest in.

See here for some good reading on the subject Bobistheoilguy

Other than that, I would stick with the manufacturer recommendations.

Things to consider are oil breaks down over time. Gas, or water that gets into the oil forms acids which increase engine wear. The cellulose, or paper in your oil filter also deteriorates over time. I have a pickup which I drive 2000 miles, or less per year. I went with synthetic oil, and use the best oil filter I could find. (Mobil 1 oil filters) I feel relatively comfortable changing my oil once per year. I am considering going longer, but I want to find more information about the safety of doing so, before I extend the interval. If you use regular oil, I would recommend every 6 months. I know that seems excessive, but engines are very costly to repair, or replace.

Absolutely NOT.

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SECTION DELETED - UNWARRANTED RUDENESS

The 1 year interval is NOT the MAXIMUM life of your oil. It is the point at which it begins to degrade. You will find counterclaims. With absolutely no evidence to back it up and plenty of worthless gimmicks they may have bought in the past, they will claim oil degrades at some other point in time--probably their favorite 3 month interval because they are still trying to justify their religiosity. Again, oil BEGINS to degrade after 12 months. It also degrades if it exceeds its mileage rating. That isn't to say 11 month oil is the same as fresh oil. Is your 11 month old TV the same as one out of the box? Probably not, it may even have a scratch or two, but I wouldn't think it's so far gone as to warrant replacement SOLELY due to age. And yes, everything degrades immediately--but it is immaterial before 12 months and unless you are a chemical engineer, you shouldn't be speaking to the subject. I'm not trying to speak to the subject specifically, I'm speaking to idiots themselves speaking to it and con auto techs and marketers doing the same.

I ran into this issue with Dex-Cool. It says replace after 150,000 miles or 5 years. Obviously, this is a huge gap and 150,000 miles doesn't even seem worth mentioning in the manual. So I worked on trying to figure out the answer. I came up with a rather simple solution that I think works and is in line with most experiences. It's not so clear-cut with oil, but I used a similar method. MATH.

A modern car rated for 7500 miles per oil change or 1 year should be perfectly fine for 4000 miles or two years...this is exponential decay in action. The point at which the fluid becomes practically worthless should be LONGER than its shelf-life, and that number is needed to estimate the decay rate. You might not prefer expired milk, but that doesn't mean drinking it will kill you--or even that you'd notice. If you do not feel comfortable with that, change it AFTER one year, not before. If you still feel that is too long, change it only after your oil life indicator says your oil life is less than 50%.

It is not mandatory ( in the US) , it is just foolish and dumb not to . A major reason is the low driving mileage . Low mileage implies short trips which is the most severe conditions for motor oil. Short trips promote water build up in the oil ; A long ,hot drive gets the oil hot and gets the water out. Many engine oil additives are added to prevent corrosion by this water. Taxi cabs can run 100,000 miles without oil change because they run so much they never get water condensation . It was done with Super Permalube (not synthetic) , an Amoco product, in the late 70's. Before internet and may not have been published anyway. Railroad diesels do not change oil, they basically run all the time so no condensation ; They do require testing for dilution by fuel and they do require that additives be replenished from time to time. But if you want to gamble a $ 2500 engine replacement against a $ 25 oil change , roll the dice.