How about Pure Tung Oil. The sun will not hurt it. Will not peel. It will soak in and protect the wood. Over time it may, as in years, need another coat. This can be done without dis-assembly.
I did my truck 6 years ago and last month I rubbed a little more Tung Oil on it. Brings out the grain of the wood and has a natural look.
I bet thats were the kero/diesel/motor oil recipe came from. Soaks right in deep easy to repeat. Not exactly factory but it is 'period' correct for working trucks and daily drivers
