Absolutely do NOT use an old adapter. If your 5080 card came with a triple 8pin to 12v-2x6 you need to use that adapter at a minimum. If your power supply is modular and you can pickup effectively a 12vhpwr cable that connects to the modular sockets on the power supply, ensure it is a NEW 12v-2x6 (H++) rated cable (almost all of them have "H++" on it somewhere). These have slightly beefier cables as well as the connections for how the pins connect into the socket (giving the connection more surface area).
Edit:
Even with all these things, I (as an engineer) am not a fan of the 12v-2x6 or 12vhpwr cables in general, especially on these 50XX series cards. There is no proper load balancing with overload limits on almost all the implementations. With very few exceptions, none of the 50XX cards can monitor the amperage going down the 6 wires individually to ensure they do not exceed the amperage limit of the physical cable (which is around 9 - 10 amps depending on the wire gauge). If any of those 6 wires and/or pin connections to the card are even slightly loose, have a bad connection, have a bad crimp, have a hard bend, internally have a broken strand, or anything else creating higher resistance for the electrical current to flow down the wire, the power will instead simply use the other wires to send the current, and possibly exceeding the amperage limit of the wire. And as I said, almost no card has voltage/amperage monitoring on the individual pins as they are effectively just taking the 6 pins and immediately tying them all together as a common source. I would go as far as to say these are all fire hazards just waiting to happen on any card that does not have individual amperage monitoring on each of the 6 pins/wires with overvoltage/overamperage cutoff circuitry.
See the below image for how all six wires under the new designs just connect together and the example of one of the few cards (the 5090 Astral) that has proper overload sense capabilities as it does not tie the six pins immediately together at the header:
Edit 2:
If you really need to use this and do not have an Astral or another card that has proper voltage/amperage monitoring of the individual pins, you absolutely need to buy a DC clamp multi-meter that you can use to check each of the six wires (it also means the six wires need to be able to be separated enough from each other so you can isolate each wire and test them and not fused together in a wireloom/bundle). I believe several youtube videos are out there that show people performing these exact measurements. A decent DC clamp multi-meter will probably run you a minimum of $75 (and goes up from there to the industry standard Fluke's which will cost around $200-500 depending on the exact model).
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