Don't know if this is of any help.
According to bentley, P87.7
They state for A/c from '86 on the following:

R-12 capacity: 50.75oz

Refrigerant Oil:
Evaporator 2.02 oz
accumulator .3 oz
high pressure hose .3 oz
low pressure hose .3 oz
compressor 5.07oz

CHRIS STANN wrote:

> The dudes at the local A/C shop put 3 cans of R134 in my system and called
> it quits.  It beats having window down, but it's not even close to what I've
> seen on this list so far.  The compressor is whining worse than my x-wife
> with cramps.  I don't think they put enough R134 in it.  They also told me I
> have a leak in the system.  Well, after two months I haven't seen any
> reduced cooling efficiency.  I also have the '86-up drier mounted
> upside-down because it would not fit any other way and I think that maybe
> hurting the system performance.  I have not replaced the expansion valve
> either.
>
> ----- Original Message -----
> From: Tom Smith <TSmith5041@aol.com>
> To: <vanagon@GERRY.VANAGON.COM>
> Sent: Tuesday, August 15, 2000 8:16 PM
> Subject: Fwd: amount of freon in ac system
>
> > Recently retro fitted to R134a with rebuilt compressor, rebuilt Refridg
> lines
> > with barrier hose, new dryer, expan valve & ester oil. Went with a charge
> =
> > to 80% of R-12 charge. Cooling OK but there's no icicles hanging from the
> > ceiling. Does anyone have any experience with the proper amount of R-134a
> to
> > use.
> >
> >
> > Tom Smith
> > 83.5 Westie w/87 AC System
> > Charlotte, NC>>
> >
> >