WD40 Causes Fin Rubber to Soften

While replacing an older turbo fin on a v2 drive I noticed the rubber was much softer around the mast base area than a new fin. I am assuming this was due to WD40 being used over many years. Hopefully WD40 is OK on the drive plastic parts but it appears to have adversely affected the rubber fin. So I need to be careful applying the WD40, or just find another water displacement spray that is not petroleum based.