I also use an MS661 and love the saw. But I run out of power on Oak and logs over 24 inches. I generally quarter the logs first, which has two advantages: 1) cutting the widest width when the chain is sharp and 2) the later cuts are narrow when chain is duller, but have proportional more power to push harder without bogging down the saw. This allows for better chips than dust. The slower you go, when power is limited, the more heat that is created and with the smaller chips, the chain gets dull quickly. I sharpen every 8 to 10 cuts on 8' logs.
Note about rip chain. Do not get Grandberg rip chain. The concept of grinding off partial parts of teeth is flawed. Their chain is not on par with the concept of saw blades (ATB or TCG) where each tooth cuts a different section of wood. All they are doing is put the load on a few teeth to do the work, but has the friction of the partial ground teeth going through the Kerf. This is not the same concept as skip chain where the whole tooth is removed.
I did a thorough review of their chain and of course, they did not post it. I have two of their chains and tested it against the Oregon 72RD. Done 1000's of bd ft to compare in multiple hardwood species. The 72RD is superior, cuts better and lasts longer.
Love the MS661, starts well after sitting for months, and the new M-Tronics is awesome. If I had to do it over, I would have sprung for the 881 Magnum series. weight is not a factor when on the log. I end up pushing way too much with the 661 to the point of bogging down, which dictate the speed and time through the cut. It is more work in the end verses an 881, harder on the chain, bar, etc. Get the Max power you can get for wide hardwoods.
Other tips: since taking off the chain in a grandberg mill is a hassle (have to take off the mill), I sharpen on the saw. I bring both files and battery operated Dremel with chainsaw grinding bits. I also use a lighter wt oil with the pump wide open to provide maximum oil. The eco friend saws of today do not throw enough oil for a 24" inch cut in oak. I built and tried an additional oil supply at the bar tip, just inside the tip sprocket, but turned out to have limited gain. I end up using Stihl Platinum oil (which is 40 wt) plus Stihl Winter (blue jug) that is 10 wt. I get a ratio around 20 wt, which flows well around the bar. BTW - The orange bottle, Woodcutter, is 30 wt. See the MSDS from Stihl on their oils.
Final tip: Since each log needs a flat cut with an external rail-set first to guide the first cuts, the height has to be changed frequently between that and final thickness cuts, I have height block for 4/4, 5/4, 8/4, etc that I put under each end of the mill when changing heights. This provides consistency of cut thickness as I switch thickness heights for each log. I do not have to mess with the number gauges, which can be troublesome from end to end tightening.