Motor Oil: 15W40 vs. 20W50

100000 Km is just getting the engine broke in now adays!

I don't see why you would need to unless it having low oil pressure problems. I have ran a few vehicles to over 250K miles with proper maintenance schedules and I used the recommended 5W30 type oil in them until the day they went to the scrap pile. The engines where still good but the vehicles just fell part around them!

100K miles on a modern engine that was properly maintained is not all that much. Most modern engines if properly maintained can easily push 300K - 500K or more and still be running strong however the vehicle they are in usually will not make it that far. Its the engines that get abused and neglected that typically die early.

Consult the manufacturer. A critical issue in specifying the appropriate oil is the ambient temperatures where the vehicle is being used. Another issue is what specification your service provider has on hand...He may want to sell you what he has, rather than what you want or need.

Right on. If you're running this vehicle in a low-temp environment, then SAE 20W-50 is too thick to provide proper lubrication.

First, as a side note, it is very unusual for a car to use 15W40 oil. This is typically used in diesel truck engines (is this a gasoline fueled vehicle?)

Having said that, if the engine is working perfectly fine on your current oil, there is no real reason to change. Changing really shouldn't hurt anything, but if it's working, you may want to leave something alone that isn't broke.

Unless you live someplace where it is ridiculously hot, you're already running too heavy an oil. The heavier the base stock oil (the first number), the higher the parasitic friction losses from it.

I had a 2.5 liter S-10 that I ran on Castrol 5W-30 for 290,000 km that towards the end, leaked more oil than it burned; less than 1/2 litre in 5,000 km oil change interval. Its end came from a drunk driver (not me) and not the mechanical demise of any oil-wetted part.

Use what your owner's manual tells you unless there are extenuating circumstances.

They do make oils for older vehicles with the same SAE specs but with slightly different additives. Keep whatever the manual recommends but choose an oil for high mileage vehicles if you feel it is necessary.

To start off, we need to know what kind of a vehicle you are talking about. Engine oil does a lot more then lubricate the engine. It also seals the piston rings, cools the engine and lubricates the moving parts. 15W40 oil is a very poor oil to use in any current model of gasoline engine, since it gives very poor lubrication at start up, and is too thick to properly lubricate most modern day gasoline engines.

Most gasoline engines built in the last 15 years, have very light tension piston rings, and a very tight fitting piston. The thick 15w40 oil does not give proper lubrication on cold start up, and will cause increased piston wear, the thick oil will also be too thick for the light tension oil control rings to scrape the oil from the cylinder walls. This allows the oil to creep into the combustion chamber and caused short catalytic converter life, along with carbon build up in the combustion chamber. 20-50 oil is even worse for everything that I have written. I do not know of any modern day engines that specify a 20-50 oil.

By far the best lubricant for any engine is the lubricant that the engine manufacturer has specified for it's operation. There is no one size fits all engine oil.

I have also been told to use a 20w50 engine oil by a motor spares company

who was very honest with me and even referred me to another spares company as he only supplies factory spares of the same vehicle. I still have about 2L of 15w 40 over can I mix the two oils together to use?