Sounds as tho you have some Ryobi 2 cycle attachments? If so, than I think the 4 cycle head is a good addition. 4 cycle isn't new for Ryobi. They are reliable. I wouldn't worry about losing sump.

I have the Troy Bilt TB475 which is a 4 cycle 26cc engine. From the looks of it, it is the same engine as the Ryobi. Double check to verify.
Being said, I have it for a few years, no used commercially, but I do use it very hard. I use it to wack tall weeds, wack all the way to the ground, bog down the engine a lot, I use it very rough. It hold up well. It has a sling inside that whip up oil, If it is up side down, the piston will continue to whip the oil anyway, I think it is ok, I had turn it around just to see and don't see it stalling. That the extend I tried. I never turn it up side down for extended period of time.

It has power, I bought a Honda 31cc HHT31SUNBA. I thought it should be much stronger. One time I have to wack in very dusty enviroment, so I decided to take out the Troy Bilt. It start in 2 pulls after months sitting, very strong. I was very surprised that it stood it's ground next to the Honda.

I think Ryobi is the same trimmer and if so, I am going to give it a thumbs up.

Honda is a lot quieter. If it is same as Troy Bilt, check Lowes, My trimmer is on close out sale here cheap, may be it can fit with your attachments.

They run inverted just fine. It's a myth that inverted running hurts the oiling. My Ryobi is probably ten years old and one of the best trimmers I have ever had. Troy Bilt and Ryobi are the same like Yungman said.

I wouldn't buy a new one, they are plenty of cheap returns on ebay for little of nothing. I got one for 15 bucks plus shipping and a two stroke for 6 bucks plus shipping (the two stroke had the recoil rope pulled out, other than that it was brand new). watch the shipping most know they will get little for them and rip you on shipping. Do your research and read for good results. I got three 4 strokes and 4 two strokes all for less than 150 bucks. They all work great.

Well, I just picked one up off the bay for $125 brand new....just the powerhead but that's fine, I believe the older straight-shaft attachments are beefier. Always wanted a 4-stroke but they were quite pricey. Will be a nice compliment to my 2-stroke Ryobi which just keeps on going.

Good for you, I think you will be happy with it.

Thanks for the all the help.