Here's one:

If it has a quart capacity, how many pennies can you put into a empty piggy bank?

You can't put any - it wouldn't be empty anymore.

You all are right on the oil thing. I don't think DS was :)

I think you can put one in an empty piggy bank, because after you put one in it isn't empty anymore.

^Yup. :)

What property do the numbers 1, 2, 6, and 10 all have that no other whole number has?

They are all three letters when spelled out?

Yep. :) I like that one.

Two more:

What has six legs but walks on only four?

What do reindeer have that no other animals have?

I remember the 3-gallon/5-gallon thing from Die Hard.

^Baby reindeer.

I have to think about the first riddle some more though.

Someone riding a horse or elephant or donkey (or other 4 legged animal)

Why did the orange juice factory worker lose his job?

Yep! :)

He got canned?

Maybe because he couldn't concentrate.

He spent too much time navel-gazing.