From US 101 near SFO, take the San Bruno Avenue exit and follow San Bruno Avenue West in the opposite direction to that indicated by the Rental Car Return signs. The first gas stations on this road are the most expensive, but there’s cheaper fuel at the Shell and 76 gas stations in half a mile at the intersection with El Camino Real. After fueling, head back along San Bruno Avenue to SFO and follow the Rental Car Return signs. Keep your fuel receipt to show the rental clerk.
If you’re on I-280, taking the exit for San Bruno Avenue towards SFO leads to the same intersection with El Camino Real where the Shell and 76 gas stations are.