Oil paint like any other paint; watercolor and acrylics come in grades or levels of quality. There is Student grade, Artist grade and Professional grade levels in paints. Years ago I think paint had a better quality of pigments to them. Through the years I believe the paints in all mediums have less quality to them. I suggest buying a higher grade of pigments if offered. Do not buy “Why pay more” brands and expect the pigment quality and brightness of pigments to be high standard. Over time and exposure to light will fade the colors a lot. If you are buying for elementary level art then it is okay.
Recently artist friends have been saying they are having trouble blending different brands of oil paints [tubes of one brand and mixing with another]. This is a good question? I started a very large oil today and I was mixing brands and had no trouble. So, not exactly sure of their problems. It could be Heavy Handed approach, or using too much thinner. Possibly the surface; a cheap canvas can be hard to cover as well. Pre-gessoed canvas are not exactly the best surfaces to paint on unless gessoed again.
TIP: Do not use turpentine to dilute paint. Use Linseed oil. There are paint thinners, paint extenders available on the market made for oil paints and even acrylics vs. diluting with water.