I want to ask about the oil change of turbodiesel engines. We've been using fully synthetic in our 2015 Mitsubishi Montero Sport. After how many months or kilometers should we have it changed? We only use the vehicle when we go out, which is very rare since I work abroad. Is it okay to have an oil change every six months or every 10,000km? Is that okay for the engine?
Quite frankly, there isn't a clear answer to your question without taking your used oil to a laboratory for chemical analysis. Doing so will tell you how long you can go before you need to change the oil. Short of that, any answer will just be quoting the lubricant's sales brochure.
In general synthetic lubricants are deemed better as there's more that goes into producing them. This is also partly why it costs more, too.
One of the selling points of fully synthetic oil is that it can keep your moving parts cleaner for longer, as it has fewer impurities. That being the case, you can extend service life for a bit longer with fully synthetic oil rather than if you had been using mineral or semi-synthetic oil. The caveat is you extend the oil change interval at the expense of foregoing the added protection of using synthetic oil in the first place. Remember, over time impurities will increase as your engine wears down.
Another thing to note: Synthetic lubricants provide better wear protection compared to mineral-based oils due to the differences in how they are manufactured. Extend the oil change interval beyond a certain point and you defeat the purpose of going synthetic to begin with.
Better resistance to heat breakdown is also another advantage. This is more critical in a turbocharged engine like your Mitsubishi Montero Sport than a normally aspirated engine. The turbocharger at full bore generates quite a bit of heat—some of which will make its way to the engine oil to accelerate its breakdown. If you don't usually drive hard enough to be on full boost most of the time, this may not be as big a concern compared to someone who does.
So, how much longer can you extend it?
I can't really tell you exact numbers for your vehicle, but from personal experience, I have been able to extend certain types of oils for several thousand kilometers further than the vehicle manufacturer's recommended interval without any issues.
I also recall the fine print in one of the advertising materials from a synthetic oil brand which claimed there was virtually no wear to a test engine which followed its manufacturer's recommended service intervals. Having said that, if you want absolute engine wear and tear protection then change oil as recommended by auto manufacturer. Longer service intervals are possible, but we're not making any claims as to how much longer over the recommended schedule.
Thanks for writing in.