I've compiled a list of what 20th- and 21st-century authoritarian leaders of nation states studied at the post-secondary level. I have not yet fully surveyed every region. I'm sure I must have made mistakes, and I'd appreciate corrections.
Most authoritarian leaders do not appear on this list, largely because they did not pursue a post-secondary education. Also, I have not included interim leaders or leaders who served for less than two years. Crucially, I'm leaving out the multitudes of authoritarian rulers who got their highest level of education at a military academy.
I classify leaders based on only their latest educational program. E.g., if someone studied Engineering and then Business, I'm just putting them in the latter category. A leader is listed twice if he (all the leaders in this list are male) studied two disciplines concurrently.
In some countries it's unclear whom to classify as the leader. In the case of Iran, I have considered both the Supreme Leader and the President to be leaders. Iran is the only country (so far) for which I've considered two national leaders.
I'm considering each national ruler as the leader of only his home country and not of countries that were subjugated by that home country. For instance, I'm not treating any Soviet ruler as a leader of Hungary. A key limitation of my method is that since I have examined only nation states, I have not included colonies, even though colonies were under authoritarian rule.
I found seven authoritarian leaders who received doctorates: Askar Akayev (Kyrgyzstan, Physics), John Magufuli (Tanzania, Chemistry), Laurent Gbagbo (Ivory Coast, History), Syngman Rhee (South Korea, History), Ilham Aliyev (Azerbaijan, History), Hendrik Verwoerd (South Africa, Psychology), and Thongloun Sisoulith (Laos, 'History of International Relations').
Most popular field: Law. Second most popular: Engineering. (I'm not counting rulers who studied engineering in military academies.) Engineering seems especially popular among leaders in Communist and formerly Communist countries.
Law (27): Jean-Claude 'Baby Doc' Duvalier (Haiti), Vladimir Lenin and Vladimir Putin (USSR or Russia), Fidel Castro (Cuba), Rodrigo Duterte (Philippines), Engelbert Dollfuss and Kurt Schuschnigg (both of Austria), Ante Pavelić (Croatia), Getúlio Dornelles 'Gegè' Vargas (Brazil), Juan María Bordaberry (Uruguay), Joaquín Balaguer (Dominican Republic), Ion Gheorghe Maurer (Romania), Manuel Estrada Cabrera (Guatemala), King Hassan II (Morocco), John Vorster and F. W. de Klerk (both of South Africa), Habib Bourguiba (Tunisia, concurrently with poli sci), Babrak Karmal (Afghanistan, concurrently with poli sci), Paul Biya (Cameroon), Ali Bongo Ondimba (Gabon), Moktar Ould Daddah (Mauritania), Lee Kuan Yew (Singapore), António de Oliveira Salazar and Marcello Caetano (both of Portugal), Zulfikar Ali Bhutto (Pakistan), Slobodan Milošević (Federal Republic of Yugoslavia), Kaysone Phomvihane (Laos, did not finish).
Engineering (20): Leonid Brezhnev (USSR, metallurgy), Mohamed Morsi (Egypt), Saparmurat Niyazov (Turkmenistan), Xi Jinping, Hu Jintao and Jiang Zemin (all 3 of China), José Eduardo dos Santos (Angola, along with radio communications), Agustín Pedro Justo (Argentina), Gustavo Rojas Pinilla (Colombia), Islam Karimov and Shavkat Mirziyoyev (both of Uzbekistan), Rahmon Nabiyev (Tajikistan), Nursultan Nazarbayev (Kazakhstan), Kurmanbek Bakiyev (Kyrgyzstan), Isaias Afwerki (Eritrea), Mahmoud Ahmadinejad (Iran), Viktor Yanukovych (Ukraine), José Napoleón Duarte (El Salvador), Sukarno (Indonesia), Miguel Díaz-Canel (Cuba).
Physics (1): Askar Akayev (Kyrgyzstan, doctorate).
Chemistry (2): John Magufuli (Tanzania, doctorate), Yen Chia-kan (Taiwan).
Biology (1): Abdiqasim Salad Hassan (Somalia).
Radio Electronics (1): Pol Pot (Cambodia, didn't finish).
"Technical Education" (1): Hassan Sheikh Mohamud (Somalia, Master of Technical Education, Bhopal University; he also attended, without completing, an MBA program).
Economics (8): Charles McArthur Ghankay Taylor (Liberia), Andrei Gromyko (USSR), Mátyás Rákosi (Hungary, specialization in external trade), Alexander Lukashenko (Belarus, concurrently with history), Emomali Rahmon (Tajikistan), Yoweri Museveni (Uganda, along with political science -- thesis on Frantz Fanon), Yumjaagiin Tsedenbal and Jambyn Batmönkh (both of Mongolia).
Management, Accounting, or Business Administration (4): Faure Gnassingbé (Togo), Anastasio Somoza García (Nicaragua, business administration), Sooronbay Jeenbekov (Kyrgyzstan), Pavel Filip (Moldova, also has a earlier degree in engineering).
Labour Administration or Labour Relations (2): Teodoro Obiang Nguema Mbasogo (Equatorial Guinea), Siaka Stevens (Sierra Leone).
Forestry (1): Nông Đức Mạnh (Vietnam).
Medicine (8): François 'Papa Doc' Duvalier (Haiti), Bashar Assad (Syria), Félix Houphouët-Boigny (Ivory Coast), Hastings Banda (Malawi), Mohammad Najibullah (Afghanistan), Agostinho Neto (Angola), Mahathir Mohamad (Malaysia), Gurbanguly Berdimuhamedow (Turkmenistan, dentistry).
Physical Education (1): Pierre Nkurunziza (Burundi).
History (7): Heydar Alirza oglu Aliyev and Ilham Aliyev (both of Azerbaijan -- Aliyev the younger has a PhD), Alexander Lukashenko (Belarus), Laurent Gbagbo (Ivory Coast, doctorate), Syngman Rhee (South Korea, Princeton doctorate concurrent with seminary studies), Nguyễn Phú Trọng (Vietnam), Mohamed Abdullahi Mohamed (Somalia).
Political Science & International Relations (3): Kassym-Jomart Tokayev (Kazakhstan), Habib Bourguiba (Tunisia, concurrently with law), Babrak Karmal (Afghanistan, concurrently with law), Thongloun Sisoulith (Laos, doctorate in the "History of International Relations").
Psychology (1): Hendrik Verwoerd (South Africa, doctorate).
Philosophy (4): Kwame Nkrumah (Ghana, MA at the University of Pennsylvania, also studied with A. J. Ayer in London), Ruhollah (Ayatollah) Khomeini (Iran, studied philosophy at seminaries), Mohammad (Ayatollah) Khatami (Iran), Burhanuddin Rabbani (Afghanistan).
Teacher's College (1): Daniel arap Moi (Kenya).
Theology (7): Stalin (didn't finish), Jozef Tiso (Slovakia), Grégoire Kayibanda (Rwanda), Syngman Rhee (South Korea, concurrent with doctoral studies in history), Ruhollah (Ayatollah) Khomeini (Iran, studied philosophy at seminaries), Ali Hosseini Khamenei (Iran), Akbar Hashemi Rafsanjani (Iran). **Kwame Nkrumah obtained a theology degree at Lincoln University before taking an MA in philosophy.
Enver Hoxha of Albania studied "natural science" at the post-secondary level, but I don't know what field of science he studied.