When it comes to using white, oil painters are faced with two choices: to paint with a titanium (or titanium/zinc) white,
1. Lead white is the warmest white available to the oil painter (see illustration above). Because of that warm cast lead it can play nicely with light values and weaker tints, the kinds of delicate passages you find in many skin tones and neutral colors. Especially if you paint with a lot of classic earth colors.
2. Lead white paint is less opaque than titanium white. Unless the white in question has been unduly adulterated by its manufacturer. At first you might consider lead white’s weaker tinting strength to be a bad thing but a lower tinting strength can become an advantage when attempting to replicate traditional effects produced by Ye Olde Masters. Effects such as such as overpainting, scumbling, or the laying in of a semi-transparent color on top of an earlier passage. Often, modern titanium white is too strong or too blue and overwhelms the color it is mixed into. Often it makes that color appear, well, chalky.
3. Lead white exhibits a distinct thixotropic property as you move it around with a brush or palette knife. Lead white tends to glide in a unique manner while it is in motion. When the motion ceases the paint drops or freezes. And lead white can transfer some of this effect into another color it has been mixed into. The titaniums I have painted with do not exhibit this behavior. And finally, lead white tends to be stiffer than most titanium whites and is less likely to level out as the paint film dries. This retains a crisper impasto.
4. Lead white will gently accelerate the drying of the oil film and strengthen the film as it does so. Without going into the chemistry of it all, the lead is a through-drier so it can speed things up without the structural dangers normally associated with using a surface drier such as cobalt, or other metal driers which pull in oxygen from the air above the film.
First, mix a tiny amount of ochre paint into a generous amount of titanium white. This will shift the cool bias titanium pigment has towards the warmer cast of lead. Just a tiny amount of ochre will do. Mix it in thoroughly using a clean palette knife on a clean surface. The slightest addition of a second color will send the white in the wrong direction. (You are just trying to shift the white from cool to warm. Compare your mix against unmodified titanium white. You’ll see how little ochre is required.)
Next, you need to reduce the tinting strength of the titanium in your titanium/ochre white. To accomplish this, you start by mixing some linseed oil into a pile of finely-ground marble dust. (aka, calcium carbonate). Use a hand muller on glass if you have them, or a substantial palette knife on a clean surface if you don’t. Exert a fair amount of pressure as you mix everything together because it must all be well incorporated before the next step. (BTW, marble dust is inexpensive and available at most art stores. Or it can be ordered online.) The consistency of your final oil and calcite blend should equate the consistency of your titanium/ochre white. Now, begin mixing a little of the oil and calcite blend into your titanium/ochre white. As you increase the amount of calcite you are lowering the opacity of the titanium. (As a point of historical fact, Velazquez often worked calcium carbonate into a number earth colors to affect their opacity. Much of the transparent beauty found in his limited palette comes from this trick. You can use you oil and calcite mixture for the same purpose).
And finally, to emulate the impasto effect lead white imparts to a brush stroke, try incorporating a small amount of artist-grade beeswax. (You will find that very little wax is needed to mimic the peaking effect of lead white.) The wax creates a shorter pull to your paint mixture and thus your mock lead white will sustain sharper peaks and striations. Good enough for impasto work. I recommend you add the wax on your palette as you need it and not incorporate it into a tubed mixture. That way you will always have the option of working with a short or long mock lead white.
You will likely want to experiment with different proportions of these additives to find your preferred mock lead, but once you find it take note for future reference. You can then make a large batch and tube it up for later convenience. Sealed properly, your mock lead white should last as long as any other oil paint.
Have fun, and keep painting!