We have been using Claessens Finest Double Oil Primed Belgium Linen for ten years. It is an all purpose linen, 11.5 oz. per square inch with 91 threads per square inch.  Oil paint bonds on a molecular level to an oil primed surface as opposed to having to cling to a plastic or acyrlic surface. It is used for plein air painting, portraits, figurative and still lifes, etc.
 We mount it on the finest Baltic Birch Plywood. Birch Plywood is rigid enough to protect the painting from bending or polked holes and yet very light weight for field painting.  Sizes up to 12"x16" are mounted on 1/8" birch and panels larger then that are on 1/4" birch and we can make any size panel you may wish.