Our brushed DC motor is rated for 3-4 amps and can be overloaded for 8+ amps. We are currently changing the brushed motor for a brushless (BLDC) motor, rated at 3.5 amps. How much can I overload the BLDC motor? I think much less, than the brushed motor. Am I correct?
tl;dr A BLDC motor may have a lower overload current than a brushed motor
When you overload something like a motor, there are several different mechanisms by which Something Bad could happen.
In a brushed motor, the brushes themselves will have a limit, which does not of course apply to a BLDC motor.
You are unlikely to be able to push enough current through the motor to break it mechanically, through too much torque. However, watch out for excess torque on things you add externally.
You could overheat the windings. With a steady current, this depends on the cooling of the motor. With a single pulse of current, it depends on the \$I^2t\$ of the pulse. If you keep the pulse duration short enough, then the amount of heat it dumps in the windings will be small.
The last, and often under-appreciated, limit on the motor current is demagnetisation of any permanent magnets in the motor. In a conventional brushed DC motor (small ones at least, less than a few kW), they are usually engineered so that the current surge taken at switch-on does not damage the motor. As that tends to be an order of magnitude more than the running current, almost everybody ignores it as being too big to be relevant, too big ever to be reached when powering it from the rated voltage.
BLDC motors on the other hand cannot be started or run by simply connecting them to a large battery. The controller always applies some form of current limiting. This may mean that BLDC motors do not have the same wide margin between running current and demagnetisation damage current. If the motor has a maximum current specified, then I would advise sticking to it. If you want to go above it, then get more data from the manufacturer, or buy a motor and test it to destruction.