An old time painter once told me that before metal ladders they always used BLO and turp cut 50/50 to preserve their wooden ladders.
I have recommended this to many who buy planks from me for their backhoe trailers. Many have used this and it seems to work for them.

If you read a can of Linseed oil it will say "stays tacky" but I thought that was only non-boiled (raw) linseed oil...