Okay, I got into my files and found the original JBL card that came with my L100s: "Maintaining JBL Furniture Finishes" (Pub. CA10-4, February, 1969).
On the back it describes the procedures for
OILED FINISHES
Oiled Walnut (WX), Rosewood (RX0, Danish Teak, etc.
and in the second paragraph it intones:
The cabinet should be rubbed with a mixture of three parts boiled linseed oil and one part pure gum turpentine. Apply a liberal amount of the mixture over the entire finished surface of the enclosure. In ten to fiftenn minutes wipe off the remaining oil with a clean, dry cloth.
So there's the official recommendation from JBL in 1969.