May one make/use a Menorah of seven branches?
It is [Biblically] forbidden to make a Menorah which resembles the Menorah in the Temple, [and is valid for use in the Temple]. Accordingly, one may not make a Menorah of seven branches, but rather of 5, or 6, or 8 branches. This prohibition to make a seven branch Menorah applies even if it is made of non-gold metals, and even if it does not contain [the features of the Biblical Menorah such as] the goblets, buttons and flowers, and even if it is not 18 Tefachim [1.4 meters] high. [It likewise applies even if one makes the Menorah into a different shape than that of the Temple, such as with half circle branches, or triangle shaped branches and the like. Those Shuls which make/own a seven-branched metal Menorah are to be protested until they remove a branch or add an eighth branch.
A non-metal Menorah of seven branches: It is permitted to make a non-metal Menorah which contains seven branches. Thus one may make a seven-branched wood, earthenware, [glass or plastic] Menorah.
What use was it to have pure oil if the Menorah itself was impure?
The Chashmonaim made a wood coating for the Menorah branches, which prevented the oil and candles from becoming impure.
 Michaber Y.D. 141:8; Rambam Beis Habechira 7:10; Avoda Zara 43a; Likkutei Sichos 20:169 [printed in Shulchan Menachem 3:275]
 Maharik 75; Birkeiy Yosef 141
 See Shach 141:35-36; Chidushei Rav Akiva Eiger ibid; Pischeiy Teshuvah 141:14
 Seven branched Menorah with eight candles: Some Poskim rule that one may make a seven branched Menorah which will hold eight candles. [Chacham Tzevi60, brought in Yad Efraim 141]
 Michaber ibid; Maharik Shoresh 75 in name of Rabbeinu Yitzchak of Krubel
The reason: As a metal Menorah remains valid for use in the Temple even though it is not made of gold. [Shach 141:35] Likewise, the Menorah remains valid for use even if it does not contain the goblets, flowers, and buttons, and even if it is not 18 Tefach high. [Shach 141:36]
A gold Menorah which does not have the flowers etc.: Some Poskim suggest that one may make a gold Menorah of seven branches so long as it does not contain the goblets and buttons, being that a gold material Menorah is invalid if it does not contain these features. [Implication of Shach ibid, brought in Bechor Shur on R”H 24, Rav Akiva Eiger 141, Pischeiy Teshuvah 141:15] However, other Poskim clearly rule that it is forbidden to make a seven-branched gold Menorah even without these features. [Bechor Shur on R”H 24, brought in Rav Akiva Eiger ibid; Pischeiy Teshuvah 141:15] Practically, one is to be stringent regarding a questionable Biblical prohibition. [See Maharik ibid; Birkeiy Yosef 141; Pischeiy Teshuvah 141:14]
 Tevuos Shur on R”H 24, brought in Pischeiy Teshuvah 141:14
Other opinions: Some Poskim lean to permit making a seven branched Menorah which contains circle shaped branches. [Mishnas Chachamim on Avodas Kochavim p. 64, brought in Pischeiy Teshuvah 141:14]
 Birkeiy Yosef 141; See Maharik ibid; Devar Moseh 1:122
 Shach 141:35; Implication of Michaber and Tur ibid
 The reason: As only a metal material Menorah is valid for use in the Temple. [Talmud; Rambam Beis Habechira 7] Thus, all other materials may be made into a Menorah of the same shape as the Temple. [Shach ibid] However, see Tevuos Shur on R”H 24, brought in Pischeiy Teshuvah 141:14 and Rav Akiva Eiger ibid that even an invalid Menorah may not be made to replicate the Temple Menorah
 Avoda Zara 43a; Kaf Hachaim 670:1