Okay, so I'm obsessed a little about whether Mobil Clean 5000, one of the lowest cost oils on the shelf, could have a Group III base stock hiding in the bottle, since it has been hinted at here before. Found a potential clue. As previously mentioned, the MSDS for MC5000 describes the fluid as Base oil severely refined (page 6 under heading "Contains"): http://www.msds.exxonmobil.com/psims/AlternateFormat.aspx?DocumentID=577121&DocumentFormat=RTF The key is in knowing how XOM interprets both the term "severely refined" (as relating to mineral oil) and what they consider as synthetic. Do they follow the post lawsuit guideline and allow the term synthetic to encompass Group III mineral oil (hydrocracked) or do they still adhere to the pre-lawsuit protocol where only Group IV PAO and above is deemed as synthetic? I found this page on their Australian website that indicates they only consider Group IV PAO and above to be worthy of being called synthetic: http://www.mobil.com/Australia-English/LCW/Audiences/Synthetic_V_Mineral.asp About 1/3 to 1/2 down the page there are four paragraph headings titled Hydroprocessed Oil, Severe Hydroprocessed Oil, Semi-Synthetics, and Synthetics. This appears to give insight into how XOM classifies base oils. So from this, it would seem to follow that the "Contents" description in the MC5000 MSDS as Base Oil Severely Refined would indicate Severe Hydroprocessed as described on the Australian XOM page, which would be Group III. Not an outright definitive statement encompassing MC5000's base oil, but given the above it makes sense that XOM would call MC5000 a conventional oil even if it is Group III, i.e. severely hydroprocessed.