Just found this recipe in my file. Maybe it could help someone...
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Homemade oyster sauce

259g bottle oysters
1 T. water
1 t. salt
soy sauce
1/2 T. dark soy sauce

Drain oyster and reserve the liquid. Mince oyster and place in a saucepan. Add water and reserved liquid and bring to a boil. Reduce heat, cover and simmer about 10 minutes.
Remove from heat, add salt and cool completely.
Force the mixture through a fine sieve into sauce pan.
Measure the liquid, adding 2 T. soy sauce to each 1/2 cup.
Add dark soy sauce and bring to boil. Reduce heat and simmer gently for about 7 minutes. Cool to room temperature and pour into a sterilized jar. Seal and store in the refrigerator. This sauce can be kept for several weeks.

Note: If bottled oyster is not available, a far substitute can be made following this recipe. Do not use smoke oyster or any other flavored preparation.
Make approx. 1 1/2 cup.