There are many many reasons cars are not as reliable as they used to be. Complicated electrical systems, engine parts made increasingly of plastic, individual parts that can’t be repaired and require the replacement of an entire component (I.E. Your door handle breaks and the only way to fix it is to replace the entire door panel.).

But the light oil issue is a major one for drive trains. Many engines weren’t originally designed for the ultra lightweight oil and their seals don’t hold up over the long term.