1991 BMW 525 leaking oil ascending unit

On BMW if the oil sending switch leaks, it's most likley cracked, replace with a new one, just use the right size socket, I cant remeber the size, last time I did one was a few years ago.

The early production cars had a problem with the casting leaking at a crack forming at a casting edge, to be sure you might consider removing the entire oil filter housing and inspecting it. The sending unit for oil pressure is a hexagonal shape and takes either a 27 or 30mm socket, no special tools required. ♥

I looked for an oil leak for 1 year and finally steam cleaned the engine. The Oil pressure switch is located next to the motor mount on the DRIVER's side of the engine across from the oil filter, near the front of the engine. It is recessed into a cavity. To remove the connector Snap wire, use a pocket knife blade or pointed small screwdriver to pull the stainless wire off. SAVE IT! (practice on a connector on top of the engine first if in doubt) Pull the pull the connector off. You need a 24 mm DEEP WELL socket to remove and replace properly. If you don't have a deep well and just a regular 24 mm socket and plan to buy a new switch and 24 mm deep well, then snap off the plastic part and remove with standard depth 24mm socket. Order a new switch at Autozone for about $12. They are all the same. Also purchase a 24 MM deep well socket to re-install. Wrap the new switch threads with teflon tape or suitable sealant and start threads by hand. Tighten switch with socket until snug. DO NOT Over Tighten. Install stainless steel snap wire back on to connector and snap connector in place over the switch. Start up and check for leaks. This trouble prone oil switch can leak and pressurize the cable and wire tube and send oil everywhere around the engine. As far as the starter motor! You look for leaks for months and can't determine the source. Change this switch first! Quit wasting oil and fix it. It's not that hard!