As I mentioned before, one of my hobbies is a car and I'm always on the lookout for car related articles. I came across an article online about "The Proper Timing To Change Engine Oil" and want to share it here. I guess many people might think that we should change the oil very often. It is said that the timing is either when a car runs 15,000km or when you keep it for a year. That means changing the oil frequently is not correct. The engine oil has many roles, which it protects the sliding portions like bearings with greasing and keeps smooth engine motion. On the other hand, it is exposed to the heat, mixed with humidity and is affected by burning gas. Hence, they cause the oil to deteriorate gradually and it will not be able to show its original performance. It will finally prevent engines from working normally. That's why we need to change the oil. The use situation has big affect on the oil deterioration. In the case that you use a car every day to go to work or go shopping, it has a tendency to earlier oil change. If you often get on the highway, the heat and load by a gear with high speed rotation can deteriorate the oil quickly. So, it's hard to decide when the best time to change the oil is. In the end, the article said that we should follow the maker's recommendation because they set the bar called "Severe Condition" which it's highly unlikely that troubles happen no matter what. In my case, I always change the oil every regular maintenance.

You know who/whom to buy this for --------------------------------------------------------------------------------- Are both(who/whom) correct above? Does it mean "You know who/whom you buy this for."?

What is the difference between delusions and illusions ? and is one positive and the other is negative ? Or both of them can be both depending on context ? * How to ask my questions as a native speaker ?