Don't know much about the linseed oil but when my truck was done 25 years ago I put a marine varnish on it and even with the bed mostly Covered with a toneau cover it peeled off with the sun the underneath lasted longer. Wounld defiantly need a uv resistant varnish .but I don't think a varnish can tolerate years of sun exposure without a redo every now and then . May be easier to oil.
