Linseed oil is just one of the two oils used in virtually all oil-based finishes, including paints. Chemical "driers" are added to speed the cross-linking (polymerization) of the oil molecules.
Tru-Oil is an oil-based finish with a fair amount of resin and wax added. It will stick to the linseed oil fine if you rough it up with a Scotchbrite pad first. Apply it heavily, allow to sit for 10-15 minutes and wipe completely dry with a clean lint-free rag. Very little finish will stay and coats will build slowly. There is no need to sand between coats except to clean up runs.
Never use old oil, even if it has not been opened, as shelf life is poor. The oil starts to polymerize when tiny amounts of Oxygen come in contact with the oil. Interrupted polymerization causes the long chains to make weak bonds that inhibit Oxygen from breaking and then bonding with when applied and open to air.
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