20% time has always meant different things depending on who you talk to. It's likely that you're just talking to different people.
I took copious amounts of 20% time in my time at Google (2009-2014). Usually I'd start a 20% project with neither my manager's knowledge nor his approval; if it looked like it had legs, I'd let him know about it and ask what he thought. I never had a manager outright forbid me from working on a 20% project; responses ranged from "You should consider this your main project now; it's critically important that we understand this area" (along with a spot bonus for delivering on it) to "Well, you can work on it, but you are unlikely to get credit for it come promo time." In general, as long as I got my work done, my managers didn't care what else I was working on.
Was your workload such that you could put in a reasonable number of hours on your 'work' and still have 20% of your time for other projects? Or was 20% time just putting in an extra 20% on top of what other developers (who didn't do 20% projects) would do?
Over my career there, yes, I could put a reasonable number of hours on my work and still do 20% projects. There were short periods of time when I was asked to "bank" the time and focus entirely on my main project, eg. I might work straight for 5 weeks on my main project and then take a week straight of 20% time once it launched.
This suited me fine - usually the way I took 20% time was to fit it into time periods when I didn't really have much else to do or I was bored with my main project. And splitting it up like this let me focus more intently on both of them, which helped in delivering.
Cool, sounds like it worked out for you about the way I would expect. I suppose it may depend some on your team, I have heard some people say it is just 20% extra work.
I took copious amounts of 20% time in my time at Google (2009-2014). Usually I'd start a 20% project with neither my manager's knowledge nor his approval; if it looked like it had legs, I'd let him know about it and ask what he thought. I never had a manager outright forbid me from working on a 20% project; responses ranged from "You should consider this your main project now; it's critically important that we understand this area" (along with a spot bonus for delivering on it) to "Well, you can work on it, but you are unlikely to get credit for it come promo time." In general, as long as I got my work done, my managers didn't care what else I was working on.