You are correct. Do NOT use petroleum oils or especially brake fluid. The mineral oil is made especially for suspension systems and is common to a variety of oddball cars (Bentley, Citroen, etc.) You MUST use only mineral based oil in your suspension system. I've not seen it in the usual big auto chains and have usually just ordered it from Fastlane or in a pinch gotten it from the dealer. It's not cheap but seldom needs to be replaced.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold