When I used non-synthetic, I'd have to add about a quarter to a half quart between every oil change, now that I've switched to Amsoil synthetic (past three oil changes), I haven't had to add any at all.

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1983 300D (parked for four years)
2012 VW Sportwagen TDI Manual
2001 Miata SE
1962 Chevrolet Corvair Rampside