Oil front of inter cooler etc

Hi

been working through the jobs on my 300tdi 90 whilst I’ve had time on my hands.

when I bought it it had a lot of oil on the front of the inter cooler (ie the side that faces the road) all around the power steering reservoir and down on the engine mounts.

any ideas what could have caused this? It is really thick. I am asking as I would like to work out if it is evidence of an issue that was fixed or something I should look for? I have degreased so anything new should be visible.
Thanks.

youll have to keep an eye on it could be a porous intercooler ,leaking hose ,pas res or pipe

Thanks that was my thoughts about de greasing it all as I would see fresh deposits. Was just surprised to see it on front of intercooler.

I thinking of replacing all off the engine bay hoses anyway.