From the TSB 9501:

Porsche 928 S MY 1983-1986

Refrigerant

Type                   grams     w/ or w/o auxiliary air
12                        1050         w/o
12                        1200         w/
134a                    860          w/o
134a                   1030          w/

 

ND Oil 8:

Type                    ml / cc

12                         270 (+/-20)
134a                    300 (+/-20)
 

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Porsche 928 S4-GT MY 1987-1989

Refrigerant

Type                   grams     w/ or w/o auxiliary air
12                        1050         w/o
12                        1200         w/
134a                    860          w/o
134a                    1030          w/

 

ND Oil 8:

Type                    ml / cc

12                         270 (+/-20)
134a                    300 (+/-20)
----------------------------------------------------------------------

Porsche 928 S4-GT MY 1990-1991

Refrigerant

Type                   grams     w/ or w/o auxiliary air
12                        1050         w/o
12                        1200         w/
134a                    860          w/o
134a                   1030          w/

 

ND Oil 8:

Type                    ml / cc

12                         120 (+/-20)
134a                    130 (+/-20)
 

----------------------------------------------------------------------

Porsche 928 S4-GT MY 1992-1995

*** the  TSB does not state this. Compressor is different 10PA20, but this is what I know. See WSM Page 87-129 for a 1993.

Refrigerant

Type                   grams     w/ or w/o auxiliary air
134a                    860          w/o
134a                   1030          w/

 

ND Oil 8:

Type                    ml / cc

134a                    160 (+/-20)
 

 

====

I checked my GTS: it holds 860 gr R134a and 130grams oil. So it says on the conversion shield. No rear AC.

Theo

===

Up to 1989:

The system requires 950 grams refrigerant (no aux/rear AC) and 1150 grams with rear AC. This is based on R12 refrigerant. When R134a is used, a 80% charge must be used.


Oil is 280 +/- 20 ml in the system. R134a must use synthetic oil (Ester)

====

Here's some of my reference notes on oil capacity:

- Porsche Technical Specifications, 90-91 S4/GT, 92-93 GTS, says:
Page 74, air conditioning, refrigerant oil in compressor: 280 +- 20 cm3 Densoil 6

- Porsche Work Shop Manual, Volume 4, page 87-90 says:
Refrigerant oil in Compressor:

Type 6E171 280 +- 20 c.c. Densoil 6

Type 10PA20C 120 +- 20 c.c. Densoil 6

- Page 87-129 says:
As of MY 93, For R134a, Compressor type 10PA20C, Refrigerant oil in compressor: 160 +- 20 c.c. Densoil 8