Thanks to everyone for their educated guesses. I can now deal with a shop from a position of strength. Many, many thanks.

I should haved added to my post that the substance -

1. Has a smell
2. Foamed on contact with the concrete floor
3. Is quite light in viscosity; it spread rapidly through a paper napkin

I'm going to chance it and drive the bus to a good truck repair shop at the opposite end of the industrial park where my bus is stored.

Regards,