As mentioned above, a leaking sump flange (IF that is the source!) is usually the fault of the DPO (Damned Previous Owner), who has overtightened the bolts.
These bolts should be tightened to 16-18 lbs/ft. That's all! Remember that 10-12 lbs/ft is 'hand tight', the torque you can exert using only hand muscles. Use a torque wrench! Many such wrenches can't measure such a low torque, but the 'beam' wrench can, is cheap and very reliable!
When the DPO heaves on the sump bolts " to make sure they don't leak", they will distort the thin sump flange, and cause it to 'bell' around the bolt holes, crushing the gasket and making a leak inevitable, not just for them but for for all future owners, as the bell prevents the flange compressing the gasket along its length. When the sump is off, and the flange cleaned, inspect it with a straight edge and a strong light behind to look for belling. If present, use gentle hammer strokes against a flat surface to straighten them out. For a fuller explanation see: https://forum.tssc.org.uk/topic/108-correcting-the-sump-flange-to-prevent-leaks/#comment-730
John