Intake Gasket? Yes, I suppose that could be the source of the leak.

The O2 Sensors see the lean condition and set about to alter the mixture by
adding more gas -- this happens by extending the injector timing. The
injectors open X (where X is a distance) every time they fire. The variable
is the time period that they remain open. If open for 500 milliseconds, then
you get a certain amount of gas, if open for 750 milliseconds then you get
more gas at the same throttle setting, and this makes the mixture richer. (I
have no clue what the actual timing is, I selected these numbers only to
illustrate the idea.) If the O2 Sensor tells the computer that the mixture
is lean, then the injector timing is lengthened. If rich, then the timing is
shortened. If the sensor(s) continue to report a condition that the injector
timing can not correct, you will eventually get a code. Keep in mind that
the O2 Sensor doesn't directly control the injector timing, it only reports
the fuel content of the exhaust stream immediately as practical after the
exhaust valves. The combustion process requires there be a trace amount of
unburnt fuel relative the amount of air. The sensors merely report to the
computer what the content is, and the computer looks at other inputs and
makes changes to bring the O2 sensor reading back to what is expected.

If you had a problem with only one of the sensors, then I would not hesitate
to think the sensor itself was the fault. But, yoiu are telling us that both
sensors are reporting a fault, so I would be looking to the fuel delivery
system -- low fuel pressure, that sort of thing -- or too much air -- leaky
hose or gasket.

On the fuel delivery side, there is a vacuum operated fuel pressure
regulator. The fuel pump delivers (for example) 60psi constantly. At idle,
you might only need 30psi to get the fuel needed, but at 70mph, you would
need every bit of that 60psi. If the pressure was low because the regulator
was letting fuel return to the tank that is actually needed at the
injectors, then you would have a lean condition. If you look at the injector
rail(s), you should be able to locate the fuel lines (yes, there are two.)
One line is bringing fuel to the rail, the other line is after the pressure
regulator and is sending fuel back to the tank so it can be used
later.Pressure regulators seldom fail where they remain closed which results
in high pressure at idle, they generally fail in a way that lets fuel go
past them when the pressure should be held as high as possible.

If you drove around town for several days and there was no code reported,
then went out onto the highway for a high speed trip to the next city and
the code popped up, then I think I'd dial in on the pressure regulator. Of
course, the fuel pump itself can deliver low pressure too ...