Linseed oil won't swell these gaps shut.
Other than new planking, I'm not aware of any remedy.
Don't use a filler or caulk!
Most people tend to just leave them.
On one canoe that I restored that had historical significance and needed to be as original as possible, I colored the inside of the canvas a brownish color to lessen the appearance of stark white canvas in gaps up to 1/4" .