£500 aint too bad for an ATF change on a D3, the filter and atf fluid would make up a good chunk of it. Insist that they drop the pan would be my advice. I used an Audi Dealer to do 3 ATF changes on my last D3 without issue but I would insist on a mastertech carrying out the work rather than some apprentice that started last week.
A ZF service centre is the place to go if you were ever in Germany at some point, very reasonable and serious attention to detail according to any customer reviews I've seen.

I think the 500 was just an oil change not dropping the pan and changing the filter

As far as I remember, correct me if I'm wrong please, oil is £200 (£20/l), new seal and filter £100 ish.

From tips oil is £34 + vat per litre 😬

Its going into the gearbox specialist tomorrow for an oil and filter change. I asked about lifegaurd 8 but they said why do you want that its a 6 speed and should have lifegaurd 6. I have read about lifegaurd 8 and a software update. What do you think ?

Unless you're having issues with your gearbox, I'd stick with the Lifeguard 6.

Some later ZF 6HPs specify Green Lifeguard 8. You need to check the label on it and compare with EKTA.
http://www.realoem.me/Audi/RDW/A8/2006/291/R/3

below is the snippet for my HYT transmission, depending on gearbox serial, code and software revision...

№ Code Title Note Quantity Model data
6-speed automatic gearbox with interaxle differential, ----------6-HP-19---------- 3.0/3.2ltr., 4.2 ltr., FSI GZV,HRK,HKL,, HKX,HYT,JAY,, HYU,JNM,HKM,, JNL



(1) 09E300036A 6-speed automatic gearbox with interaxle differential, with software: 4.0 ltr., 4E0 910 156 B 1 GVK
2 09L300036M 6-speed automatic gearbox with interaxle differential, software: 4E0 910 156 F 1 GZV
2 09L300036MX 6-speed automatic gearbox with interaxle differential, software: 4E0 910 156 F 1 GZV
2 09L300038H 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 M 1 HRK
2 09L300037G 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 A 1 HKL
2 09L300037GX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 A 1 HKL
2 09L300039C 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 A 1 HYT,HKL
2 09L300039CX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 A 1 HYT
2 09L300037GX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 A 1 HKL
2 09L300039M 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 N 1 JAY
2 09L300039MX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 N 1 JAY
2 09L300041T 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 Q 1 JNL
2 09L300040K 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 Q 1 JNL
2 09L300040KX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 Q 1 JNL
2 09L300037H 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 B 1 HKX
2 09L300037HX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 B 1 HKX
2 09L300039D 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 B 1 HYU,HKX
2 09L300039DX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 B 1 HYU,HKX
2 09L300040L 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 R 1 JNM
2 09L300040LX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 R 1 JNM
2 09L300042A 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 C 1 HKM
2 09L300037J 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 C 1 HKM
2 09L300037JX 6-speed automatic gearbox with interaxle differential, software: 4E1 910 156 C 1 HKM
(2A) G 055005A2 atf (automatic transm. fluid), filling quantity:, see workshop manual, GN 4000001>> 4000597, GN >> 95253 1.0 ltr., 9,0LTR., yellow, yellow X;1 GZV,HRK,HKL,, HKX
(2A) G 055005A6 atf (automatic transm. fluid), filling quantity:, see workshop manual, GN >> 4000597, GN >> 95253 20ltr., 9,0LTR., yellow, yellow X;1 GZV,HRK,HKL,, HKX
(2A) G 055162A2 atf (automatic transm. fluid), filling quantity:, see workshop manual, GN 4000598>> 4005000, GN 95254>> 100000 1.0 ltr., 9,0LTR., blue, blue X;1 GZV,HRK,HKL,, HKX,HYT,JAY,, HYU,JNM,HKM
2A G 060162A2 atf (automatic transm. fluid), filling quantity:, see workshop manual 1.0 ltr., 9,0LTR., green X;1

I briefly had Lifegard 8 in my gearbox but they said it was already on the latest software, so couldn't update it. I think that meant that it wasn't optimised to accept the LG 8 oil.

I ended up having issues between 1st and 2nd, and eventually had my mechatronic unit rebuilt, the fluid changed (conventionally) to LG 6 and the adaptations reset to cure the clunking.

If you're interested, search on here for "clunking" and you should see my thread.

Mine was done yesterday and was refilled with lifegaurd 6 with a new filter. Total inc vat was £330. Its made no difference but as it wasn't broken b4 i guess thats good news. They said the oil was very clean and the magnets had minimal on them so all is good 😀 🤞

Sounds like a result and quite reasonable considering the cost of ATF fluid and a filter. I got mine changed every 60K miles and it certainly did no harm over the years.