Double idle gear type (i.e.: Pete Jackson)? .008 will work fine.

Single idler type (i.e.: Milodon)? Believe it or not, they recommend a strip of newspaper between the cam gear and idler and another between the drive gear and idler for sizing. So, you figure a piece of newspaper is about .003 thick. Two would equate to .006 ish.

I agree with the boyz, why have the extra noise? That's why I use a Milodon (not meant to be a sales pitch), but very quiet. But then I added the blower...go figure.