Is it really true that it is better to exercise at a lower intensity for fat burning? i heard if you exercise at 60-70 percent intensity for a longer time this is best. otherwise your body doesnt use your fat stores to fuel the workout.

Preferentially: using your body's fat store for energy is not so much a function of the intensity as it is duration. The typical example is a non-world class marathon runner who expends 70-75% intensity for a long period, between 3-5 hours. Burning carbohydrate stores and switching to lipid metaboism starts after 45-60 minutes of continuous exercise at a moderate to moderately intense activity.