Remember this famous saying: "Lubrication is of the essence." Guys know that finding the best lubricant for intercourse is one of the most important things they can do for their sex life but put it off. It's much more interesting to study sexual positions, eh?
Well, consider this: all your bedroom technique and performance is absolutely meaningless if your woman is in pain. In another link on Female Libido and Pain, I cover the many, many reasons women can experience pain during sex (besides accidently knocking a lamp onto her head).
One of the most common is vaginal dryness, because females of any age can experience some vaginal dryness during certain times of the month that can make prolonged intercourse painful. Furthermore, 40% of post-menopausal and/or post-hysterectomy women experience atrophic vaginitis, a drying and thinning of the vaginal walls that occurs from declining estrogen levels.  The microtearing of the vaginal walls can lead to inflammation, yeast infections and even bleeding in some cases.  Yes, this will be bad for your sex life.
So it's time for Lubrication 101. It's probably safe to say that most of you know more about lubing your car or truck than the ingredients in a good sexual lubricant. Well, below I discuss (in descending order) the best ingredients and lubricants for intercourse and - trust me - it's significantly more complicated than 10W30 versus 10W40.
1. All Natural. In my opinion the best products are all natural. As you'll see below, most of the ingredients in lubricants are straight out of the chem lab and the petroleum fields and most will likely bother the woman in your life one way or the other. Consider, for example, the top four ingredients in Astroglide Natural: a naturally occurring sugar (xylitol), an aloe leaf extract, potassium ascorbyl tocopheryl phosphate (an antioxidant blend of vitamins C and E) and pectin (as in fruit). It is also very slippery, long lasting and non-staining, the three critical properties for females everywhere. These type of lubricants are starting to rapidly gain market share and, I believe, will soon dominate.
2. Silicon. Silicon lubricants tend to be very slippery, relatively long lasting and, most importantly for many, petroleum free. This makes silicon-based lubricants the preference of many couples. However, silicon definitely has its down side, the foremost being that it can stain clothing, sheets and any other fabrics that you come into contact as your are flailing around the room in ecstasy. The silicon stains can be cleaned, but it requires a certain amount of art and science that some people will not have the patience for. Again, though, many couples swear by it - silicon lubricants are used all the time in industrial applications for a reason! - and probably the classic brand is probably K-Y Sensual Silk K-Y's Silk is one of the most slippery and slidey of all lubricants and has good staying power as well. One caution is that it is watery in feel and can drip easily upon application. An alternate K-Y silicon brand is K-Y Intrigue, which is also long-lasting and a little less watery. It has more of an oily feel which some couples prefer. It is, though, about double the price as Silk and has less of a slick feel. Personal preference I guess...
3. Glycerin. Most of the inexpensive lubricants out there, a la Walmart/Target variety, are glycerin-based. Glycerin is a "sugar alcohol" that is slick, slippery and fairly long-lasting. However, glycerin can break down into sugars and so many experts believe that it can lead or exacerbate yeast infections.
4. Parabens and Petroleum Products. Many of the cheaper lubricants, including the ones that contain glycerin, contain propylene glycol which generally comes from the petroleum byproduct propylene. Propylene glycol is nearly ubiquious in modern societies, being placed in toothpastes, foods, cosmetics and so on. Propylene glycol has survived a number of cancer studies. However, it has definitely been linked, in its airborne form, to increased allergies and asthma.  Why? Because it is an irritant. Furthermore, it has been shown to be very irritating to the skin at even low dosages.  So why not put it in products that come into contact with delicate vaginal and penile tissue, right? Wrong! Many compounding pharmacies actually create propylene glycol-free formulations for those who are sensitive to the stuff.
So, remember, "Sex shouldn't hurt." Well, that is, unless you're into that sort of thing...
1) AM FAM Physician, 2000, 61(10):3090-3096, "Diagnosis and treatment of atrophic vaginitis"
5) Journal of The American Academy of Dermatology, 1991, 24:90-95, "Propylene Glycol Dermatitis"
6) PLOS One, 2010, 5(10):e13423, "Common Household Chemicals and the Allergy Risks in Pre-School Age Children"