HTB will do it fine - only time I have ever seen one fail (except for 1000's of hours use) is when they are never greased. I use a bit of teflon grease (like nulon l80) and have noticed cooler gearbox temps (to the touch) in similar operating conditions.
No affiliation to Nulon, use Inox MX7 teflon grease if you want, but I am using it for everything now and it is keeping things going that really shouldn't be (eg I had a severely rust seized bearing in a trimmer head and after freeing it up it continues to perform when repacked with l80). Wouldn't have fared as well on straight bearing grease I don't think.
