Diamondoids are a class of cage-like hydrocarbons that are structurally similar to small pieces of diamonds. Their formation from polycyclic precursors parallels their dynamic stability. Diamondoids occur naturally in virtually every oil and condensate as well as in source-rock extracts. Their ubiquitous nature in oils and extracts of any thermal maturity level, along with their proven stability with increasing maturity, makes them useful as thermal maturity proxies past peak oil window and in the identification of mixtures of low- and high-maturity fluids. Two relationships in particular, methyldiamantane index (or MDI) and 3- + 4-methyldiamantane concentration vs. Stigmastane [5 alpha, 14 alpha, 17 alpha(H)-24-ethylcholestane 20R C29 sterane] concentration, are repeatedly used to establish thermal maturity relationships between oils and between oils and extracts and to identify mixtures of fluids with different thermal maturities. The present comparative study shows how methyldiamantane proxies from West Texas oils and extracts (sourced from the Woodford Shale) and in oil samples from northern South America (Colombia) and the Middle East show overall a very good agreement with classic maturity proxies, such as Tmax values from Rock-Eval pyrolysis and vitrinite reflectance (Vro). However, caution is advised when using methyldiamondoid-derived proxies in the presence of drilling additives and contaminants (e.g., oil-based mud or OBM) and of oils and extracts that have experienced biodegradation. The latter is probably a highly unrecognized issue when using methyldiamondoids, which can result in erroneous interpretations of thermal maturity and mixing. Based on the results of this study we recommend the necessity of a holistic, multi-proxy approach, based on the integration of geochemical screening, organic petrography, and molecular geochemistry techniques as the best possible solution when dealing with such geochemical conundrums.