Drain the mineral and use the synthetic. More importantly, once you have the AC system open, replace every o-ring you can in the system!! I bought a $25 box of green Viton o-rings at Autozone and with some silicone grease (from a scuba shop) on them, replace 15 o-rings in my '87 560SEC. I found two split ones in the compressor that caused my leak, two in one fitting, one pinched (damaged) o-ring, and several that were hard as a rock. It was worth the effort!!

Also, don't overdo the ester oil in the system. The manual for my car said 300cc of oil and with that much in there it would not cool.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered