The slippery surface racing flag, also known as the surface flag or colloquially as the "oil" flag, is a yellow flag with red stripes used in motorsports to warn drivers of localized hazardous conditions that reduce grip.  Displayed stationary (not waved) at local flag stations, it indicates the presence of oil, coolant, water, or debris on the track surface.

This flag should be displayed, for at least (depending on the circumstances) 4 laps unless the surface returns to normal beforehand. It is not, however, necessary for the sector beyond where this flag is being shown to show a green flag.

The red and yellow striped flag, or surface flag is displayed stationary at local flag stations to indicate that track conditions have changed due to substances on the track which could reduce grip or cause a car to lose control.

Generally oil, coolant, small pieces of debris or sand are the hazards. It can also be "rocked" back and forth (but not waved) to indicate a small animal on the racing surface. Many organizations will display this flag for only two laps, after which the changed surface is considered to merely be "part of the track". Wikipedia