Wood bed oil advice needed

[dads longhorn]

For 37 years I think my dad put used motor on the wood bed to preserve it. It worked, the wood is in excellent condition. For the last 5 years I've done nothing to it but wash it at the car wash. The 37 years of accumulated dust is finally coming off enough that its time to begin protecting the wood again. What to use? New motor oil? A bit flammable maybe? Another oil type that won't work against the old motor oil? Not varnish or that type of protectant. What do you recommend? Thank you in advance.
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