For cDNA synthesis from RNAs with a poly(A) tail, the Oligo d(T)23 VN (NEB #S1327) is recommended. If desired, a longer dT oligo can be used. Alternatively, a gene-specific primer can be used in the cDNA reaction in place of the poly(dT) oligo.
For RNA templates without a poly(A) tail, random primers or gene-specific primers can be used for cDNA synthesis. Addition of a gene-specific primer to the cDNA reaction may slightly increase the cDNA yield of the specific target. We have tested final concentrations of 0.1 µM, 0.5 µM, and 1 µM gene-specific primers with good performance at all concentrations, but would suggest 0.5 µM as a starting point and adjust accordingly.