**NO.** Not in the strict scriptural sense of the word Guru.



In both Vedic and Agamic Dharmas, a Guru has to be someone who is alive. I too have some friend who considers Adi Shankara as his Guru. 

But unless he takes DikshA from a living Guru he will be considered technically as an uninitiate (adikshita) only. Such mentally accepting someone as one's Guru may look fancy but does not really work. (Having said that in very rare cases, one is allowed to accept Lord Shiva or Mother Goddess as their Guide-Guru but that's an exception not a rule).


**Guru or Teacher in the Vedic Dharma:**



> The first birth is from the mother and the second at the tying of the
> Muñja-grass girdle. At the latter birth, the Savitri verse is his
> mother, while the teacher is said to be his father.
> 
> **The man who initiates a person and teaches him the entire Veda is called ‘teacher**’; whereas someone who teaches just a section of the
> Veda is called ‘tutor’, as also someone who teaches the Vedic
> Supplements.
> 
> **VDS 3.20**


So, obviously, the teacher or Guru has to be alive to initiate the student and teach him (the student) the Vedas.


The student, after being initiated, has a lot of duties and he has to stay along with his teacher in his (the teacher's) house.


> Student––**he should serve his teacher until death** and, when the
> teacher dies, the sacred fire;  for it is stated, ‘The fire is your
> teacher’
> 
> 
> 
> 12 **He should wait upon his teacher, walking behind him if he is
> walking, standing by if he is seated, and sitting if he is lying down.**
> 13 He should recite his lesson only when he is called upon to do so.
> 14He should announce everything he has received to the teacher and eat
> with his permission.
> 
> **VDS verses.**


So, it is more than obvious now that we are talking about a living Guru here.




**Guru in the Agamic Dharma:**


In Agamic DikshA SamskAra too, a Guru has to be alive in order to transfer energy, consciousness, mantras etc into the disciple. The following verse defines the Guru more precisely.




> **EkAkshmarapradAtAram** yo gurunchA vamanayet |<br> SwAnam yoni satam gatvA chandAlatvam vApnuyAt || 
> 
> ....
> 
> 
> One who insults the Guru, **who is the giver of ekAkshara** (or OM),
> gets the womb of dogs for hundred births and thereafter gets chandAla-hood (low human birth).
> 
> 
> **KT 11.74.**



So, here the Guru is defined to be the one who gives disciple the ekAkshara or the OM. Now, obviously doing so is not possible for someone who is dead.

DikshA is actually a process where the Guru gives mantra into Shisya's ears by chanting it a particular number of times. Their nearness (sAmippya) is essential in the process. Needless to say, both have to be alive in order to participate.



The next verse says, that all kinds of knowledge, conducts, scriptures, mantras are successful only if they are learned from Guru's mouth. And, there is no other way.




> PAramparyA AgamAmnAyAm mantrAchArAdikam priye |<br> Sarvam **guru
> mukhAllabdham saphalam** syAnna chAnyathA ||
> 
> 
> 
> ............
> 
> 
> 
> ParmaparA (lineage), Agamas, AmnAya, Mantras, AchAra (conducts) are
> all fruitful if obtained from Guru's mouth ( i.e directly
> learned from a personal living Guru), and otherwise not.
> 
> **KT 11.46.**


Further, it is stated in scriptures that just like a wife's sins get attached to her husband, likewise a Guru has to pay for his disciple's misdeeds. Hence, Guru, before initiating the disciple, has to test him in many ways. In fact, they both have to mutually test each other before the DikshA.


The following verse gives the amount time for which such tests are to be conducted:



> EkAvedana dvijo yogyah kshatriyo vatsara dvAt |<br> Vaishyo yogya
> stribhirvashairshchaturbhih shudra eva cha ||
> 
> 
> ...........
> 
> 
> 
> 
> The Brahmin becomes competent [for dikshA] after one year, a Kshatriya after two
> years, a Vaishya after three years and a Shudra after four years.
> 
> **KT 14.105.**


EkAvedana in the verse means one year. So, for a Brahmin disciple, the mutual test time is one year, for Kshatriya it's two years.... and for a Shudra disciple it's four years.


Now, obviously Guru only if he is alive can conduct such tests and can be the subject of such tests.



> Can I have multiple gurus (Guru Tulya) unless I get a guru where I get
> personal attention, which is not possible these days?


Well, such persons have not actually become your "Gurus". So, there is no harm in mentally accepting more than one such Saints as your mentors or protectors.

And, even in case of actual Gurus, having more than one Gurus is allowed ( I myself have two Gurus).



> MadhuluvedhA yathA bhringah pushpAt pushAntaram vrajet |<br> JyAna luvdha
> stathA shishyah guro gurvantra vrajet ||
> 
> 
> .........
> 
> 
> 
> Just like the bee, desirous of honey, moves from one flower to the
> other, a disciple, desirous of knowledge, can likewise move from one
> Guru to another.
> 
> 
> **KT 13.132**


And, similar thing has been stated in the Uddhava Gita too.



However, there can be some exceptional kinds of Gurus. Anyone or anything who (which) teaches you something is your Guru.

Lord Dattatreya is known to have 24 such unusual Gurus.



>  In the Uddhava Gita, a song embedded in the Bhagavata Purana
> (11.7-9), Dattatreya tells King Yadu about his 24 unusual gurus:
> 
>  
> 
> 1. The Earth
> ------------
> 
>  
> 
> The Earth is disrespectfully trodden on by all creatures but bears
> them all without complaint. **So from Her, I learned to accept all of
> life’s pleasures and pains with forbearance.**
> 
>  
> 
> 2. The Wind
> -----------
> 
>  
> 
> Wind is of two types, Prana refers to the internal life forces and
> Vayu refers to the external movement of air.  Prana is in all bodies
> but takes on the shape and form of the particular body it is in.  Vayu
> passes everywhere but does not remain in one place.  **So from Prana and
> Vayu I learned to avoid attachment to any place but to adapt to
> wherever I might find myself.**
> 
> 
> 3. Akasha
> ---------
> 
>  
> 
> Akasha is one and all-pervading.  **From it I learned that Brahman is one
> and all-pervading.**
> 
>  
> 
> 4. Water
> --------
> 
>  
> 
> Water purifies and sanctifies.  **From it I learned to be a source of
> purity and sanctity.**
> 
>  
> 
> 5. Fire
> -------
> 
>  
> 
> Fire burns things leaving only their essence. **From it I learned that a
> yogi should see in beings not all the false opposites like rich-poor,
> old-young, high-low, but only the Atma which is the essence.**
> 
>  
> 
> 6. Moon
> -------
> 
>  
> 
> The one and same moon appears to go through changes, waxing, waning
> etc. **From Him I learned that the one atman only appears to go through
> changes such as birth and death.**







**Abbreviations used:**


KT- KulArnava Tantram ( All verses in this text are spoken by Lord Shiva to his consort Sri Devi). Also, KT 1.100 means verse number 100 of chapter 1 of the text.
<br>VDS- The Dharma Sutras of Vashishta.




*Regarding mantras received in dreams, i will probably update the answer later.*