I removed 4 Superior Investment Cast head with steel barrels cylinders from my O-320 after 2,100 hours. I used AeroShell 15-50 almost exclusively except for using Phillips 20-50M for break-in the first 25 hours. When the cylinders were removed, the cross-hatch was still in the cylinders and they measured out as NEW spec including the choke. I used 1 quart every 25-hours for the first 500 hours then went up to one (1) quart every six (6) hours when I removed the cylinders after 2,100 hours.
I replaced the cylinders with ECI Titan Steel cylinders. I broke the ECI cylinders in on Phillips 20-50M for 25 hours. I have been using Phillips X/C 20-50 since then. After 171.6 hobbs hours since installing the new cylinders and using Phillips X/C 20-50, the oil consumption is one (1) quart every 45-hours. I have never heard of an aircraft engine going that long without using a quart. Maybe I have the only one. Report in if you can go that long without adding oil. I have not added oil between changes but instead change it a little bit early.
When oil change time comes, I have 6-quarts of X/C 20-50 left. I like the idea of the Lycoming additive. I will more than likely switch back to AeroShell 15-50 once all the Phillips is used up. The cost delta for the Phillips XC 20-50 plus the Lycoming additive was equal to the cost of the AeroShell 15-50. I purchased the Phillips X/C at A P Fischer as they are close enough for me to pick it up from them.
http://www.apfischer.com/ Aircraft Spruce is a little bit longer drive and may have price matched them. I also have a local AeroShell dealer close to my hangar but they are the same price as A P Fischer near where I live.
With the Constant Speed prop, I am ready to fly at 70 degrees F oil temp. If I use the 100+, I need to wait till I hit 100 degrees F oil temp till the prop cycles the same as it does on the 15-50 or 20-50.
Ask any oil man or engineer and they will tell you that the multi-weight oil is thinner at low temp when compared to a straight weight. When you get to an ELEVATED temperature (above normal operating temp) the multi-weight oil will be thicker than the straight weight oil.
I spend a little bit more of my hard earned money on the multi-weight oil. Based on my experience with it over the 2,100 hours, I think it was worth it. Maybe I would have had no wear with the straight weight. The cost delta is not something that I am going to worry about to experiment with on my engine. All new car manufacturers that I know of recommend multi-weight oil for their engines. I do not know of any present new car manufacturer that recommends a straight weight oil. Do you have to use a multi-weight oil in our aircraft engine to make it last a long time? NO.