There are 2 types of 6-pin to 8-pin power adapters.
One type converts from two 6-pin plugs to a single 8-pin plug. These adapters could damage your computer and should not be used under any circumstances.
The second type converts a single 6-pin plug to a single 8-pin plug. NVIDIA also advises against its use since many power supplies will not provide sufficient current over the 6-pin power cable. However, this type of adaptor could potentially support normal operation as long as the customer checks their PSU manuals and ensures that its 6-pin PCI-E rails can handle the same current rating as an 8-pin power cable, which is 150 watts.
The recommended solution is to use a power supply with native 8-pin power cables.