In the past I have used shoplights from a company named Commercial Electric that is sold through Home Depot. These lights used T8 32 W or T12 40W bulbs and aside from an occasional minor hum, I never had a problem. This same company now advertises and sells the nearly identical shop light but the fixture states T12 40W, T12 34 W or T12 25W bulbs. I have several of the older lamps in my garage workshop and I use 32 watt T 8 bulbs. I experimented by putting the same 32 watt bulb in the newer fixture and it seems to work OK. My question is, by using the 32 watt T8 bulb in the newer T12 fixture will the bulb burn out faster?
What is the difference between T8 & T12 other than the diameter of the tube if the wattage is the same?

You should always use the bulbs that the manufacturer recommends.

To answer your question, the difference between the T8 and T12 besides the diameter is the T8 was designed to be more efficient. It uses about 20% less energy for the same amount of light. For example, a 32W T8 should output the same amount of light as a 40W T12.

-To add: I would say it's 10-60% savings--depending on T8.
-T8's will provide more lumens per watt

Open the light fixture up and read the ballast. It will tell you how many and what kind of tubes it is rated for. If you use tubes that it is not rated for, you will have early tube failure or early ballast failure. Or both, it depends on the combination. My experience has been that T8s on a T12 ballast will kill the tube early, but it is hard on the ballast too.

Most ballasts (not all) that are rated for T12 tubes are not rated for T8s and vice versa. I have seen some cheap OEM T8 ballasts that were rated for one particular T12 bulb type, but the good quality ballasts that I buy for replacement are either for one tube type or the other. Work yourself away from T12 tubes, T8s are more efficient. T5s are even more so, but the cost negates it in the small quantities that individuals use.

FWIW: that number in the bulb designation, T12, T8, A19, PAR30, G16-1/2 etc, is the diameter of the bulb in 1/8ths of an inch. And the letter denotes the shape. T12 = tubular 12/8".

EPM