What OEM? Often the OEM will force a website provider to require certain crazy things like that.
For example up until about a year ago, GM required a street address on every lead form. 
Luckily now neither an address or zip is required on GM CDK sites. Although I'm still not a fan of CDK and it's "Next Gen" platform.
I don't believe a simple form field is entirely what's causing the difference between mobile and desktop conversion rates, there are many more factors at play:
1) Screen size. When I look at my analytics, my conversion rate increases as the screen size increases.
2) Browse vs Buy. E-commerce experiences the same thing, consumers tend to browse on mobile and then buy on desktop. Some of this does have to deal with ease of use.
3) Speed. Websites tend to load slower on mobile, every extra millisecond of load time decreases the likelihood someone will convert.
4) Distractions. A mobile user is much more likely to be distracted by real life happening around them or an incoming cell phone notification.
5) Mobility. Search intent tends to be different on mobile vs desktop. Users searching for "car repair near me" are looking for an instant solution to their query and will convert in-store instead of online.
