Not sure if oils will help, unless the hair is wrapped. There was a Biewer breeder here that shaved or trimmed (sorry, can't remember & wish I knew how to look for that pic) the belly part only bc it was matting up so badly.
It helps to shave the underbelly, even up to where the belly band would wrap around the bottom. It won't look funny. I shave my girls' & guys' underbelly all the way up to their chest in the summer time bc it gets so hot & humid here.
Here it is!
http://www.yorkietalk.com/forums/yor...ath-belly.html