[Timothy C. Tyler]

I like your drawing. I think the main thing is that your underpainting is more lean that what follows. I understand it matters not which type of oil is in the paint but rather how much oil you add (in which amounts) that may give problems later. I'd say use only turps to thin and no medium at all in the underpainting. As far as the oils in the paints, many of us use several brands at once with several types of oils intermixed. Other artists will use one oil (as a medium) while the oil in the tube of paint is different as well. For underpainting-staying lean is the big concern. You can always slow or speed up drying time other ways.