Oil Sending Unit Leaking

Kind of rare (I've only had it happen once myself). Those senders are all cheaply made  and crimped together with some sort of internal gasket. Mine was squirting pressurized oil out the riveted-on lug! I would throw it away & replace with new but if you're obsessive about components, hose it down with rubbing alcohol, then when its dry, sprinkle it with a small amount of flour. Start the engine and run it only for a minute. A close look will tell you where the oil is coming from.

never has a leaking sender, are you sure it leaks not between intake manifold and block ?

It looks like you use a gasket and not sealer.

Simon

What Simon said and be sure your PCV valve and breather are working properly.

Just a little positive crankcase pressure will push oil through the seam between the manifold and the block, particularly if you have the turkey tray installed.

I had a leak on the oil pressure sender but it was located at the screw where the wire connects and was clearly visible. Besides, it no longer worked correctly.

$12 bucks for a new one at Rock Auto.  

Note- with all gauges and senders that use screw-on electrical connections, you need TWO wrenches. One grabs the big nut bearing against the case and the other grabs the small one to tighten the wire against the big nut. Using only the small nut to tighten securely, you run the very real risk of pulling the guts of the gauge or sender apart, to make an 'open circuit'. It may not leak (gauges especially!) but it also may not work even if new.

Second caution- use ONLY TWO LAYERS of teflon pipe tape on all liquid senders. I've seen 6 or 7 layers used on some senders "to be sure it don't leak". Teflon tape is an insulator and if you use too many layers, the sender, which depends on grounding against the block to work, may not ground it's case. And again, you'll get an open circuit. Check all inop' senders with a grounded wire to its case before throwing away a perfectly good sender. Temp senders seem especially sensitive. My 2 cents-

Replaced the oil sending unit and no more leaks.  The old was leaking around the nut.

Part number is SP60 and made my Standard.  Cost was about $22 at the local Oreilly's.

John