The oil fill cap has a gasket. It's a flat rubber ring. If the "new" cap has an old hard gasket it would leak just like your old one.

Next item is the flame trap, which is Volvo's answer to a PCV valve. When the flame trap gets clogged with old gunk the engine leaks oil wherever the seals are weakest. That's because with the breather system clogged the crankcase fluids + vapors get hot and expand, and pressure builds. Lucky for you the weak seal was up top where it's easy to see. It can also leak from the front oil seals which are just behind the timing belt, and from the rear main seal, or from other places too. They all cost much more to fix than swapping an oil fill cap gasket. So you have to make sure the flame trap is clean.

You need to clean the flame trap and the hoses to/from it.
The information you need is in the 700/900 FAQ, at:
http://www.brickboard.com/FAQ/700-900/EngineSealsBeltsVent.htm#Crankcase_Ventilation_A_Treatise

Your '89 flame tap is located higher up than what's described in the FAQ. Between the valve cover and the fuel rail, towards the rear. You'll see a thin hose coming off it that goes to the center of the intake manifold's branches. Fat hose goes rearward and meets the main air intake hose on rear, just before the throttle body (you can see that too). Third hose comes off the bottom of flame trap and goes straight down to the "breather box" (nearly impossible to see without removing stuff first).

Using this information and what's in the FAQ you or your mechanic can find the flame trap and clean it. You might need to clean or replace the breather box too. My preferred method to access it is NOT to take off the whole intake manifold. Instead, find the FAQ's section starting with "Disconnect Idle Air Control Solenoid (IAC) hose and PCV breather hose". That's what you need to read. With the IAC out of the way you can get to the breather.

Good luck! Ya gotta get that system clean. If you don't oil leaks will plague you due to the pressure buildup in the crankcase, simply from the system being clogged.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.