I found this on the GL1800riders forum. I can’t vouch for it, because I don’t have the Honda “common service manual” but it seems more plausible than “typo.”
It still doesn’t mean 20 ft/lbs is right. I’m sticking with hand tight + a tiny nudge
“When I first saw that torque spec, I thought it had to be a typo. So I asked Honda's tech folks about it and was assured that it was correct. Seems strange, because the filter base and the crankcase surface are the same as on the 1500, and that filter specs at 7 ft. lbs. Maybe the 1800 runs higher oil pressure than the 1500 - couldn't say.
Here's what I think happened. It's not unusual for parts or specs to change during a model run, while the service manual text never gets updated. The Honda Common Service Manual explains that Honda uses two types of "small filters;" one without graduations at the base that gets torqued to 7 ft. lbs, and another WITH 12 graduations marked along the filter base - that one specs at 19 ft. lbs. (OR fully hand tightened plus one graduation - 1/12th of a turn). It's possible that the 1800 originally was designed for the graduated filter. But there have been at least 2 changes to the filter spec since the bike was released, and I don't believe the current filter # has any graduations along the base. I agree that 19 ft. lbs seems tight enough to gouge the crankcase surface with the filter base. And if you're using an aftermarket filter with the normal square-section O ring, I'd definitely not apply 19 ft. lbs to it. Just the normal spin-on till the gasket contacts the engine, then another 3/4 to 1 turn. 1/4 turn after gasket contact isn't going to provide nearly enough gasket compression to prevent loosening or for proper sealing with high oil pressure. So I'd be safety wiring mine too, if I'd decided for some reason to only tighten to 1/4 turn. But as Bill O'Reilly would say, that's ridiculous.”
It still doesn’t mean 20 ft/lbs is right. I’m sticking with hand tight + a tiny nudge
“When I first saw that torque spec, I thought it had to be a typo. So I asked Honda's tech folks about it and was assured that it was correct. Seems strange, because the filter base and the crankcase surface are the same as on the 1500, and that filter specs at 7 ft. lbs. Maybe the 1800 runs higher oil pressure than the 1500 - couldn't say.
Here's what I think happened. It's not unusual for parts or specs to change during a model run, while the service manual text never gets updated. The Honda Common Service Manual explains that Honda uses two types of "small filters;" one without graduations at the base that gets torqued to 7 ft. lbs, and another WITH 12 graduations marked along the filter base - that one specs at 19 ft. lbs. (OR fully hand tightened plus one graduation - 1/12th of a turn). It's possible that the 1800 originally was designed for the graduated filter. But there have been at least 2 changes to the filter spec since the bike was released, and I don't believe the current filter # has any graduations along the base. I agree that 19 ft. lbs seems tight enough to gouge the crankcase surface with the filter base. And if you're using an aftermarket filter with the normal square-section O ring, I'd definitely not apply 19 ft. lbs to it. Just the normal spin-on till the gasket contacts the engine, then another 3/4 to 1 turn. 1/4 turn after gasket contact isn't going to provide nearly enough gasket compression to prevent loosening or for proper sealing with high oil pressure. So I'd be safety wiring mine too, if I'd decided for some reason to only tighten to 1/4 turn. But as Bill O'Reilly would say, that's ridiculous.”