Thanks Dr. @M.Seidam, I have to split it over 2 sessions to watch the whole thing This reminds me of similar "lifetime ATF concept" that I first hear from my friend who owns a Dodge. When I first hear this, I was surprised since most mechanics recommend changing Automatic Transmission Fluid every 40,000kms or if used just for going to office maybe can extend to 80,000kms. And to add more salt, these "modern lifetime transmissions" don't come with dipstick to check ATF level or quality
After some discussions, I realize that what dealers nowadays mean is "lifetime of your car loan / car ownership" which is usually 4 years and people usually sell their cars before reaching 100,000 kms. After this 4-5 years/100,000kms the dealers are hoping you'll get their latest version under-warranty and repeat the cycle again
Now back to the Engine Oil topic, hmm if what he says is true that "Dealers put the cheapest oil to save cost, even Toyota dealers" (since he's ex-Dealer mechanic), then we need to worry about our 10,000km oil change. There is no such thing as 5,000km vs 10,000km oil, there is oil with different viscosity level (e.g. 0W-40), liter size or quarts, and Mineral, Semi-synthetic and Fully-synthetic. Usually when mechanics refer to 10,000km oil it means the Fully-synthetic as "it can last more" and slower to degrade over time and usage. And so, if dealers pick the cheapest, that would not be the most expensive fully-synthetic, which means you're not getting that "10,000kms oil" 😂
Ok now as to when best to change oil... 5000kms is a better interval 10,000 for sure. But as regular offroaders who may go once-a-week (or twice a week!), we should learn what a drop of engine oil on paper means 👇 (use the dipstick to get a good drop and let it absorb on paper for a while)
Once it gets Dark and "Middle-condition" quality, time for an engine oil change, even though you've just reached 4,000kms! Even worse, if the shape is not round, means you have some water contamination there 😦
As for the Viscosity, usually best to refer to Owner's Manual, most will refer to 5W-30. But with the knowledge that higher load + higher temperature will make the oil thinner, and that owner manual is written for "general usage of cars" + "one average setting that can accommodate worldwide situation", we can opt for something thicker like 40 or higher.
The reason is we're trying to avoid this boundary lubrication, where the oil is too thin that the metal to metal starts touching and causing wear & tear. There's an oil film between bearings and crankshaft, and if the oil is too thin too often, causing metal-to-metal friction will certainly lead to Engine Rebuild like in the shared Youtube
So my personal opinion is to get 5W-40, 10W-40, 15W-40. If you have a supercharger and climbing Sweihan every week may want to try 20W-50 for staying thick at high load + high temp