I've been in the business for 9 yrs and my guys are reliable and do great work, but are real rough with the equipment. I have a full time job in addition to the lawn business, so not being present has led to premature equipment wear. I've switched from Echo to Stihl fs70 2 cycle 2 years ago and am very pleased with the performance. However, I am considering switching to a different Stihl line trimmer and want to know how much additional maintenance and durability to expect from a rx100 or 4 mix engine. I want to up the engine displacement to 31cc (100RX or larger) but am afraid to switch to the 4 mix. My guys want me to get the 250 w 40cc engine, but I don't think it's necessary for 16/18 residential lawns cut per day. I would only consider the 250 for durability. I am looking for reliability with low maintenance and high durability. Your insight is greatly appreciated.