Follow your state and local Codes if it tells you to use type L use it there is a reason.
Type L vents are suitable for use with gas or oil. They can be used instead of a B vent, but a B vent can't be used instead of an L vent. In most cases, L vents are used with oil. Again, this is typically a double wall vent with an air space between. The inner wall is typically stainless steel and the outer wall is galvanized. Type L vents normally see temperatures of under 750F. Type L vents are tested for up to 1,000F.