If you could list the ingredients, that would likely help us decide on what might evaporate or degrade. As a first pass, I usually put a couple of layers of something like 'saran wrap' over the opening and then replace the cap.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

Theoretically the best storage is at 40 degrees in a blqack glass bottle with all air removed.

Unfortunately, that's often hard to do.

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Strelnik
Invest in America: Buy a Congressman!

1950 170SD
1951 Citroen 11BN
1953 Citroen 11BNF limo
1953 220a project
1959 180D
1960 190D
1960 Borgward Isabella TS 2dr
1983 240D daily driver
1983 380SL
1990 350SDL daily driver alt
3 x Citroen DS21M, down from 5
3 x Citroen 2CV, down from 6