In a city where both olive oil and sesame oil are called 'oil,' a person vows not to use 'oil.' Which oil is forbidden?
When local usage is split — some call X by a term, others do not — neither meaning controls. The matter is ambiguous and we cannot resolve the vow's scope definitively.
Question 2
A person vows not to eat 'grains of wheat.' Are wheat-flour products (bread, pasta) included?
Vowing off wheat grains forbids wheat in all its forms — fresh, dried, and processed as flour — since they all derive from and are ordinarily identified with wheat grains.
Question 3
Someone vows not to benefit from 'those who rest on the Sabbath.' Is a Shabbat-observant non-Jew included?
The phrase 'those who rest on the Sabbath' in popular usage covers any person known to observe the Sabbath — including non-Jews who do so — not just halachic Jews.
Question 4
A person vows not to eat 'fish.' May he consume fish brine?
Vowing off fish does not include brine or fish-oil dip, since these are derivative products not ordinarily designated by the word 'fish.'
Question 5
A person vows not to benefit from 'the descendants of Abraham.' May he benefit from Ishmaelites?
Though Ishmael was literally Abraham's son, the phrase 'descendants of Abraham' in vow-context follows popular usage, which refers to the Jewish people. Ishmaelites are thus permitted.