I just finished changing the front fork springs to Progressive Suspension units and also changing the fork oil..................
I spoke with Larry Longly (sp) from Progressive Suspension to decide on what weight oil to use. He told me that the with the GL1500 he recommended moving up to a heavier wieght oil but with the GL1800 he said I should stick with the OEM weight of 10Wt.
He also said that any brand of frok oil should be fine, no need to stick with Honda brand...................
I ended up putting 10wt Amsoil synthetic fork oil in my forks.
BTW - If you do change to Progressive units up front make sure you don;t fill the forks to the 5" mark like the service manual calls for. Because the Progressive units displace more oil it is recommended you don't fill them any higher than 5.5"