I'm no engineer, but there are some around here and I'll ask them to chime in. I'll bet the loads experienced by car engine oils are vastly higher than anything you can generate in a bike chain. And I think high bearing speeds make it harder to maintain a lubricating film.
Which bike oil is "synthetic"? They're all mostly petroleum distillates.
In real life, the precise qualities of the lubricant are less important than 1) getting it into the relevant spots; 2) maintaining some film; and 3) keeping it relatively uncontaminated by road dirt. This last is where homebrew works well, and duplicates (IMO) the most important qualities of a lube like prolink. The thinned oil flows in and drips off, carrying much dirt with it. It's clean-and-lube in one step.
And here's where the cheapness of homebrew becomes relevant, superflychief. Because it's so cheap, I don't hesitate to use it as excessively and frequently. I go through the equivalent of a big bottle of prolink in much less than a season (rain commuter) . As for time and hassle, I pour oil in the squeeze bottle to about the 1/4 level (measurement needn't be precise), top up with OMS (which I already have around for painting use), and shake a bit. Takes 1 minute, maybe.