To expand on the point raised by Joseph:
Those who adopt the leniencies of the Igros Moshe often do so without halachic sanction.
Rav Yisroel Salanter zt’l wrote (in the introduction to the sefer Ohr HaYom) that one cannot accept the leniencies of even the greatest Gadol b’Yisroel UNLESS one also accepts his stringencies (he wrote this even regarding a Gadol of the stature of the Vilna Gaon zt’l, who certainly was considered Gadol Hador and Posek Hador).
The Igros Moshe himself advanced a similar concept in O.C. 2:60, where he posits that one may conduct oneself according to the Vilna Gaon in cases where he is a minority opinion ONLY if one adheres to all of the Vilna Gaon’s halachic rulings, both stringent and lenient.
DO ALL THOSE WHO SHAVE THEIR BEARDS, RELYING ON RAV MOSHE FEINSTEIN ZT’L’S REPORTED HETER [which Rav Moshe Feinstein never recorded in any of his sefarim] (where he is clearly a minority opinion disputed by the great Poskim of the previous and current generations), ALSO FOLLOW ALL OF RAV MOSHE FEINSTEIN ZT’L’S NUMEROUS CHUMROS (STRINGENCIES)?
Examples include:
Using a Shabbos clock (the Igros Moshe [O.C. 4:60] rules that one may not set a timer before Shabbos in order to cause a melachah to be performed on Shabbos).
Listening to music (the Igros Moshe [O.C. 1:166; Y.D. 1:173; Y.D. 2:111 and 142] concludes that all instrumental music is forbidden, and that a ba’al nefesh should avoid listening to any form of singing).
Carrying on Shabbos in communities in large metropolitan areas where eiruvin have been established (the Igros Moshe disputes the validity of eiruvin in neighborhoods such as Williamsburg [O.C. 1:138 and 140; Y.D. 3:161], Manhattan [O.C. 1:139; O.C. 4:89; Y.D. 3:161], Flatbush [O.C. 4:87-88; O.C. 5:28-29; Y.D. 3:161], Borough Park [O.C. 5:28-29 and 89; Y.D. 3:161] and Detroit [O.C. 5:29]).
Wearing a tallis katan made out of a material other than wool (the Igros Moshe [O.C. 1:2; 2:1; 3:1; 3:52] advocates wearing a woolen tallis katan, and states that he himself did so even in the heat of summer. According to Igros Moshe, O.C. 2:1, wearing a talis koton made of synthetic fabric would present the following problems: 1) non-fulfillment of the mitzvah, 2) uttering a blessing in vain, 3) carrying on Shabbos).
Eating veal (the Igros Moshe [EH 4:92:2] rules that a ba’al nefesh should refrain from consuming veal).
Shaking hands with a woman (Igros Moshe O.C. I:113 and EH 4:32:9 state that shaking hands with a woman “is clearly forbidden;” “is certainly clearly forbidden.” See also EH 1:56 where the conclusion is that practically speaking it is difficult to rely on any leniency).
Attending sports stadiums and theaters (Igros Moshe Y.D. 4:11:1 rules that going to sports stadiums and theaters is forbidden due to the prohibitions of Moshav Leitzim, Bitul Torah and promiscuity [but not due to Chukos Goyim]).
(There are additional examples as well.)