98Volvo s70 heater core just replaced and now it smells like oil and there is an oily film on the inside of the windshield. Why?

What is on the inside windshield is antifreeze.Oil and antifreeze don;t mix like oil and water don't mix.The only other thing can think of is if you have a control valve to open and close its antifreeze line at least one feed or return witch would operate from the engine vacuum /pressure so vacuum to open pressure to close.This type of valve would be near the inlet and outlet part of the water lines on the fire wall going into the heater core.If the shout off valve using engine pressure shut the antifreeze off but the seal to the engine is leaking oil could get into the duct work and transfer to the wind shield in witch case look for high engine pressure PCV valve clogged engine control or AC function issues vacuum booster control unit power AST BRAKES abs brakes .You can have a vacuum test 30 to 40 psi at idle hot general normal 20 NOT BAD FOR OLDER ENGINE You just don't want 100 psi at 2000 rpms Let me know What is on the inside windshield is antifreeze.Oil and antifreeze don;t mix like oil and water don't mix.The only other thing can think of is if you have a control valve to open and close its antifreeze line at least one feed or return witch would operate from the engine vacuum /pressure so vacuum to open pressure to close.This type of valve would be near the inlet and outlet part of the water lines on the fire wall going into the heater core.If the shout off valve using engine pressure shut the antifreeze off but the seal to the engine is leaking oil could get into the duct work and transfer to the wind shield in witch case look for high engine pressure PCV valve clogged engine control or AC function issues vacuum booster control unit power AST BRAKES abs brakes .You can have a vacuum test 30 to 40 psi at idle hot general normal 20 NOT BAD FOR OLDER ENGINE You just don't want 100 psi at 2000 rpms Let me know