Still some oxidation during kegging

Looking for some help on oxidation issues while kegging. Taste was great after secondary fermentation (with a 10 day dry hop). I cleaned with PBW, sanitized with iodophor solution (all parts separate and then running it through the lines) as usual. I rack with an autosiphon making sure the cane is submerged into the keg filled with CO2. I try and minimize any splashing. I seal it, hit it with 20 psi and shake it for a minute. Then purge off (until the heavy air gushing subsides), then reduce to 10 psi. I purge again after an hour or so, and leave it at 10 psi. 24 hours later, I notice some oxidation taste.

What am I doing wrong? How much do I need to purge? How often? Is it true that the heavy loud gushing air is oxygen that needs to be purged and after that it is CO2? Is it OK to leave the serving line and cheap handheld tap connected?
Thanks!!

Hmm. Don't know about all this shaking and purging business.

I like to fill the keg with minimal splashing, hook up the CO2 with the relief valve on the keg lid open and cycle some CO2 through the headspace. That will remove any O2 that's in there before I set it to carb psi. The rest of the O2 should have been displaced out as I filled.

Do not leave the serving line hooked up. I've had bad poppet valves on that side before cause leaks. You won't be serving it until it's carbed, so why risk it?

You need to purge BEFORE you start all the shaking. Here's my routine:

Remove lid from keg. Connect CO2 line. Open valve and let CO2 flow into (open) keg for 3-5 seconds, to get a bed of CO2 in there.

Droop the line from the autosiphon into the keg. Plunge the autosiphon cane to start the flow. The output end (of the flexible tubing) is at the very bottom of the keg and is quickly beneath the surface of the beer as it fills the keg.

Once the keg is filled, I remove the autosiphon and put the lid back on the keg.

I open the valve on my CO2 tank and watch the pressure guage. As it reaches 5-10 psi, I pull the pressure relief valve on the keg to purge any remaining air in the keg. I'll do this 2 or 3 times. The last time, I'll let it stabilize at 10 psi and let it sit for a few minutes. I'll then purge one more time, and then tip the keg over and start rolling it (if burst carbing) or move it into the fridge (if force carbing over 2 weeks).

The step I didn't see in your description was purging the air after sealing the keg, but before you start shaking. If you just capped it and immediately started shaking, there's still oxygen in that keg and yes, you would have immediately and thoroughly oxidized your beer. I know - I've done it before. It sucks to realize you forgot to purge after putting all that time and effort into the beer only to ruin it by shaking it without having purged the air.

Thanks. Yeah, I may have not purged properly before shaking. The keg was full of CO2 when I siphoned in, so I thought it was fine.

So, do you have to open the valve and keep purging every once in awhile? Or once you are confident you've eliminated oxygen from the headspace, there is no need to do that?

Once you've purged it all, you don't need to keep purging periodically every few days, if that's what you're asking.