Gamblin Artist Grade Oils - Hansa Yellow Medium
If you are an a-la-prima (one layer or direct)) painter, who paints with big, bold colours, Gamblin Hansa Yellow Medium is perfect for you. It is opaque, with high tinting strength and is perfectly primary, with no hint of either red or blue in it. That makes it ideal for mixing colours.
If you paint with a more subtle, nuanced palette, you might want to also include Rembrandt Transparent Yellow in your paint box. It is also perfectly primary, but has less tinting strength. Because of this, you can get into a finer gradient of yellow content in a colour. With two yellow tinting strengths, you can either "hit it with a hammer, or with a feather", as your application dictates.
If you paint with traditional glaze oil methods, you will just need the one primary yellow - Rembrandt Transparent Yellow Medium. It is transparent enough for glazing and serves all your colour mixing needs in foundation work as well as in in glaze layers.