There are some fairly complicated nomenclatural and taxonomic issues around this genus. Whether Postia, Oligoporus, or other names like Rhodonia and Spongiporus are correct depend on two issues. First, whether Postia is accepted as validly published – it is an older generic name than Oligoporus, but there is some controversy over the validity of the publication of Postia. If it is not valid, Oligoporus takes priority. I believe most current mycologists who have looked into the issue accept the valid publication of Postia, but the issue remains unresolved.

Second, based on work by Niemelä (cited in Spirin, et al), another strategy is to divide the group into Postia, Oligoporus, and several other genera, including Rhodonia and Spongiporus. If this strategy is adopted, I’m not sure which California species fall into Postia and which in Oligoporus, but in that case Spongiporus leucospongia would be the proper name for our most common species.

Sources:

Walker J. 1996. An opinion on the validity of the generic name Postia Fries 1874 (Eumycota: Aphyllophorales). Australasian Mycological Newsletter 15(2):23–26.
http://tinyurl.com/ASMJune96

Spirin WA, Zmitrovich IV, Wasser SP. 2006. Oligoporus balsameus: rare Eurasian species plus notes on some related taxa. Mycotaxon 97:73–82.
http://tinyurl.com/spirinetal
abstract: http://www.mycotaxon.com/vol/abstracts/97/97-73.html

The separation between Tyromyces and Oligoporus, Postia, Spongiporus, etc, however, is justified in terms of both molecular phylogeny and ecophysiology. Tyromyces, which is restricted to Tyromyces chioneus and perhaps some close allies, is a white rot fungus that is phylogenetically distant from the other genera, which are all brown rotters and well ensconced in a brown rotter clade.

Sources:
Yao YJ, Pegler DN, Chase MW. 1999. Application of ITS (nrDNA) sequences in the phylogenetic study of Tyromyces s.l. Mycological Research 103:219–229.
abstract: http://journals.cambridge.org/...

Hibbet DS, Donoghue MJ. 2001. Analysis of character correlations among wood decay mechanisms, mating systems, and substrate ranges in Homobasidiomycetes. Systematic Biology 50:215–242.
http://tinyurl.com/homobasid