From this about.com link

Get the Good Oil

When brewing homemade biodiesel, good-quality used oil is the key to achieving a correct chemical reaction that results in high quality fuel.

Our years of experience—and trial and error—have proven that Chinese and Vietnamese restaurants often have the best waste vegetable oil (WVO) for making homebrewed biodiesel. And, several of our supply restaurants’ waste oil can be of such consistent quality that we can actually use the same recipe batch after batch—although we caution that this is usually not the case. We make our fuel in 5-gallon buckets one at a time to minimize waste and maximize the ease of handling and transporting the finished fuel to the vehicle fuel tank.
The Homebrew Biodiesel Recipe

Because our supply of WVO is so consistent, this is the recipe that we use for every batch:

15 liters of oil
2.6 liters of methanol (approximately 17% by volume of oil)
53 grams of lye (3.5 grams per liter of oil)

However, all new or unknown WVO must be tested before attempting to make a full batch of biodiesel. With every new supply comes a host of unknowns, so simply throwing a standard recipe together probably will leave a bad reaction and a trail of disappointment.
Test for Quality

When starting with the very first batch of waste oil from a friendly neighborhood restaurant, it’s imperative to test the quality of the oil, through either mini test batches or titration, to determine the appropriate amount lye needed for a good biodiesel reaction (transesterification). Methanol should be 20 percent by volume (e.g. 15 liters of oil requires 3 liters of methanol). In our standard recipe above, you’ll notice that we use 2.6 liters—just a tick more than 17 percent.

This amount was derived from many experimental batches. While our main reason is economy (the less methanol used, the cheaper to make), this also helps to keep the level of unreacted agents to a minimum. All virgin biodiesel has trace amounts of unreacted lye and methanol (sodium methoxide) suspended in the finished fuel, and minimizing extraneous amounts of ingredients results in purer fuel. Another method for reducing this unreacted residue is water-washing biodiesel.
Testing the Oil: Titration and Test Batches

One hundred percent virgin or lightly used waste vegetable oil (WVO) requires 3.5 grams of lye per liter to initiate transesterification. Heavily used oil is usually quite acidic and can require significantly more lye (a strong base) to jump-start the reaction. There are two common methods to determine the appropriate amount of lye needed to convert a particular batch of WVO.

Titration is a process that utilizes distilled water, lye and isopropyl alcohol to test the acidity of a batch of WVO. The results of the test indicate the amount of lye necessary. Learn how to do a titration.

Test batches are just mini transeterification reactions using proportional amounts of reactants based on a well-educated estimate of the WVO’s acidity. Experience with many different batches of oil yields very accurate assessments. For example: light colored oil will very likely require minimal lye, whereas heavily used dark WVO will most probably be very acidic and need substantial amounts of lye. Learn how to do mini test batches.

Note: We have acquired batches of oil over the years that despite our most diligent efforts, have proven impossible to coax into reaction. Time-saving hint: if the oil smells rancid or is extremely dark and/or thick, walk away. Frustration mounts quickly when trying to make biodiesel from inferior WVO.