Most Cobras in the states are driven less than 2,000 miles a year.
I always changed mine (fuel injected with conventional
oil) every year in the spring. Winter weather, didn't drive it much in the winter, lots of moisture and probably condensation inside the motor thus water in the
oil.
100 miles or 3,000 didn't matter. Change the
oil, cheap insurance.
And those with carbs get a lot of gasoline draining into the crankcase.
Now those with newer fuel injected motors using
synthetic oil have less of a problem. I would feel good about changing every 5,000 to 10,000 miles under those conditions.