Different accents/dialects applicable to the pinyin "r" sound
There have been three major ways of transcribing Pinyin r-:


voiced retroflex fricative /ʐ/
voiced retroflex approximant /ɻ/
voiced postalveolar approximant /ɹ̠/ 


In English-language pronunciation manuals, the first is closer to English measure, the third is the English red across most accents (with the caveat that it is not labialised in Mandarin Chinese). 

There are two reasons why the first one, as a fricative, has become deprecated:


Phonologically, it means that Pinyin sh- vs r- becomes Mandarin's one phonemic voicing contrast.
High friction leads to a "rather unnatural" pronunciation. 


Hence, in the words of Třísková (2008):


  [ʐ] can be considered as a free variant


... but also: 


  Lately there is [an opinion] that [ʐ], [ɻ] are free variants of the phoneme /r/ (we can [observe] a considerable variability of pronunciation). 


So what is this variation then? An acoustic study from 王祥灿 (2013) concludes that:


  PTH 辅音r变体类型分布较为复杂，易受到具体语音环境的影响，当后接i,u,e高元音时，或音节声调为去声调时，或音素 r 位于双音节中时，多表现为擦音类型，其他情况下通音类型居多。


That is to say, use of the fricative / high-friction variant is more likely:


in Pinyin ri, ru, re
in falling tone (Pinyin fourth tone)
in bisyllables / bisyllabic compounds


I would clarify that last one by saying it's more likely directly after a consonant, e.g. higher friction in 当然 dāngrán than in 偶然 ǒurán.

Dialectal/regional differences are also found for Pinyin r-, but the approximant vs fricative difference is more contextual than regional.