Question for you gents, I am looking at a Maule with an O-360-C1F and I noticed that there is a regular steel cylinder with 3 other P10 cylinders. Anyone know if it's ok to be mixing them? I have heard it's not, but I am wondering if there is a reference. The only reason it caught my eye is that it appears to have been changed without a log entry and no record of parts replaced or where the cylinder came from, which is a separate issue.

Interesting question, on the mechanical side I don't think it would hurt a darn thing to have one jug slightly oversize. Most of the engines out there have now or in the past had a weak cylinder with no problems so why would one just a tiny bit stronger cause one?
Legally, I don't have a clue.

You make a good point. The biggest issue I see is the weight difference, since the regular piston is lighter than the three P10 pistons. I know they have a max weight limit difference, but I can't find it!

Best I could find:

https://www.expaircraft.com/PDF/Lycoming-OH-Manual.pdf

In Section II; page 1-7: (Service limit table) in the note section:
"See Service Instruction No. 1243 for identification of cast and forged pistons The suffix "S" that will be found with the part number on 76966,78203,LW-10207L,W-10545, LW-11487L,W-13358L,W-14610 pistons indicates the piston weigh its within the limits specified for any group of pistons and may be substituted for any like piston on a particular engine Other pistons are manufactured within weight limits that do not require any weight controlled piston for replacement."

I guess you'll have to dig into Service instruction No. 1243 for more detailed info