Most flatbed printed signs aren't really made with cleaning down the road in mind, unless you treat them after printing.
If known ahead of time, I would say the signs should be laminated or at least liquid cleared and then a mild soap and water with a light touch from a rag will do. If an oily atmosphere is going to happen, I'd give no warranties other than a year.
In fact, you might want to consider putting a poly or acrylic face on it, then you can feel at ease about basically any cleaners they might use by mistake. I can't tell you how many times people are given specific instructions on how to maintain signs, but someone will invariably use something else and not tell anyone and then say, I only used Windex............
