Measuring the Internal Resistance of a Brushed DC Motor for Use in Speed Control (IR Compensation)
Measuring with a multimeter is OK, but what you need to do is stop turning the motor. When you do, the motor acts as a generator and the result (it's called back-EMF) messes up the multimeter ohms circuit. Even better, fix the shaft in place and put a few volts (say, 10% of normal operating voltage) and measure the current. Then compute R = V / I.

You would expect fairly consistent R and L values from the same model motor, but, as indicated above, turning the shaft while you take the reading will give you false results.