According to the web, hypromellose ( we have those drops in our medical kit at work but in all my years, nobody has ever needed them) is hydroxypropyl methylcellulose which is a synthetic polymer - a lubricant. I presume that the 0.5% is just more of that ingredient than the 0.3%
There is another thread running on this .....Agree with Diamond1409 - with regard to VISCO TEARS. I was prescribed them about 12 months ago, for daily use. They certainly were a huge help, I'm now only using them say once a week. Depending on your NHS Region / Age you can get them on prescription. Or over the Counter from leading chemists.