As he wrote in the blog, it's quoted from 日置流弓目録.
第四十六條 矢の根に塗薬の事 薬に三色有り，二味一色，一味二色入れ物の事，右の内二色は尋矢，角木，的矢，弓の矢摺にも塗るなり。
As you can see, the quotation mentioned just 薬, and there is no explanation about what kind of 薬 you should put on 尋矢, 角木, etc. I checked the original text of 日置流弓目録 in the Digital Library of the Tokyo National Museum, and the semi-modern Japanese translation of it, but there is no mention about なもみの油, either.
東京国立博物館デジタルライブラリー / 日置流弓目録
日置流嫡傳秘書 日置流弓目録
(32 pages pdf file, 5.2MB)
It's not a commonly used word, so the best way to know it is to ask him about it.
EDIT:
I found the term. The writer of the blog miswrote the name of the book. He quoted it from 日置流目録, not 日置流
弓目録.
日置流嫡傳秘書 日置流目録
(72 pages pdf file, 10.7MB)
Here's the semi-modern Japanese translation of the part (page 32. I added commmas and periods to make it easier to read).
…ぬけ薬を付まする。其薬と申まするは、おんじやくの石の事でござりまする。おんじやくと申まするは、火にあたゝめまして人の肌へつけまする石でござりまする。色はねづみ色にて、かみそり砥の様なるものでござりまする。夫をこまかに粉に致しまして、くるみの油を以て火の上におき、とろとろといたしまする様ねりまするを、矢の根に沢山ぬり付まして置まする。
And there is a note outside of the text.
なもみの油を取り、器に入置て可用とも云ふ。
I still cannot find what なもみの油 is (the most possible one I can think of is 菜籾, i.e., "rape seed oil"), but the original text says that it's just a substitute of ぬけ薬. Thus, there is no problem if you can get the original ぬけ薬.
The writer of the blog made another mistake about the interpretation of the original text, by the way. おんじゃく is not the name of 薬, but the name of stone (most likely
温石). ぬけ薬 is made from the powder of おんじゃく and walnut oil. It's not that おんじゃく is the name of 薬 made from iron powder and walnut oil, as he wrote.
EDIT2:
According to the site linked below, a theory about the etymology of オナモミ is 雄菜揉み, i.e., large namomi. Thus, なもみ can refer to a group of plants mdchachi-san mentioned above. So, なもみの油 could be "Xanthium oil". (This interpretation seems more reasonable than 菜籾.)
生薬の玉手箱 【蒼耳子（ソウジシ）】 - 株式会社ウチダ和漢薬
