The formation of sediments in biogenic and fossil heating oils as well as in their blends is a well-known problem. These deposits can plug filters and nozzles in heating systems and, consequently, cause economic losses. Polymerization and the formation of corrosive acids are possible explanations for these incidents. To study the influence of long-term storage on different heating oils (biogenic, fossil, and a 10% blend) and to investigate the changes in their composition, the oils were stored for a period of 12–24 months at nearly ambient (40 °C) and analyzed with different techniques every 6 weeks. The formation of several kinds of oxidation products was demonstrated, including ketones, epoxides, aldehydes, carboxylic acids, and furans. Size-exclusion chromatography was used to demonstrate the formation of oligomeric products of the fatty acid methyl esters (FAMEs) (up to pentamers). Short-chain (C1–C6) carboxylic acids were quantified with ion chromatography, and larger carboxylic acids were indicated by mass spectrometry. The first recorded experimental evidence for a coupling reaction between a FAME and components of the fossil oil, namely, such containing the nitrogen heterocycle indols, is described. Cross-coupling products between biogenic and fossil compounds were detected using Orbitrap ultrahigh-resolution electrospray ionization mass spectrometry.