NJ,
I am almost certain you can't get a .325 sprocket for 020 to drive a .325 chain. I'm pretty certain spur sprockets are the only ones available for 020.
The reason you have to change oiler gears is because on the 1/4 in. is an eight tooth sprocket to run the oiler gear, the 3/8th's Picco is a six. I put a pic up so you could see the difference, the one on the left is the 1/4 in with the oiler gear, the one on the right is the 3/8 th's, I didn't have a gear that was off a saw for that sprocket for the pic, you can see the difference in the two sprockets.
Larry