Ours started to leak in 2014 on the way north to Nova Scotia and it got progressively worse in the next few months. On the way south, we got tired of drying up the little cubby hole every day with a sponge and stopped at a marina in Georgia where with much, much effort they dug out 1 of the 3 stripes of packing and put one Imperial-sized (is this what you call non-metric?) strip of the stuff that is commonly available in the US, which I think is flax-based. The old packing looked similar to the new "grey" US packing material.
The fix lasted waterproof for only a couple of weeks but the leak never got as bad as previously.
In Florida we received the metric packing from Amel, which is white, and which appears to be made from synthetic materials. It got installed and the white screw tightened as much as our experienced Amel mechanic felt it was possible to tighten it.
By the time we reached La Rochelle there again was leakage, even if modest. The leakage had started almost immediately after we left Florida. The Amel mechanic tightened several turns (i.e., much more than what in Florida was believed to be "safe"). The thing has now been 100% waterproof for over six months.
We are grateful to Eric of Kimberlite for the advice he gave us in Huntington to keep a spare set of packing and a spare white screw in a ziplock bag next to the autopilot ram.
SM2K Nr. 350 (2002)
docked in Barcelona
We had this problem on the way to Tahiti in 2014. The leak practically flooded the aft cabin and the water ended up in the compartment under the floor just outside the aft head. The gland was repacked by a local guy who managed to strip the thread on the nylon nut and it still leaked. I managed to stop it by taking everything apart, removing one piece of packing (there are 3) and putting in some small diam rope so I could get the nut further down on the thread. That worked and lasted until New Zealand where I bought some new packing (3/8") and a new nut from Amel and now it doesn't leak at all. I used the PVC pipe I use for the c-drive seals to push the packing down into the stuffing box..it was an easy job.