IMHO, Sachs 6 and 7 speed freewheels are a great choice--- but their internals often need to be serviced because they were over greased in the first place, and the original grease often turns into a "peanut butter" like consistency.

Doing two things on an annual or more frequent basis will significantly increase the life of a freewheel. First, remove the freewheel from the hub and remove the sprockets from the freewheel body. Scrape off the encrusted crud and road grime, and scrub them with whatever you prefer, WD40, mineral spirits, Dawn dish detergent, etc. The cleaner you keep your sprockets the longer they will last.

The same is true for your chain. Remove it from the bike and clean it to look new or nearly new. Finish by lubing it off the bike, allowing the lube to penetrate the roller bearings. Finally wipe and clean as much of the lube off the external surfaces as possible. We want our chains to shed dirt, not retain it. They last significantly longer by cleaning them more often.

Just my two cents.