As long as the oil is at the minimum required it would be more than enough for the maximum endurance of the aircraft, even if it was burning abnormally high levels. It would need to be burning extremely high levels to cause any issues. Regardless of that hypothetical would you refuse to go because the Oil quantity was at a minimum for a long sector? Unless you were going to a port with no engineers then possibly, otherwise I wouldn't think so. At the end of the day the 737 minimum is still 75% of capacity anyway, it's not that low.