This time I took the oil to 53hrs airborne time
I am now using just W80, with Camguard, 1 bottle spread over about 14 bottles of oil. Dropped 15W50 completely.
I don’t know of anyone wasting 20 quid plus postage to try that 
Yes it should be zero, I think.
If the oil has been in the engine, it won’t be zero; it will collect some stuff from the bottom of the sump. It is for this sort of reason that I warm up the engine before draining the oil and time 1 minute after opening the drain, before taking the sample.
What readings do you get if you send them a sample of unused oil straight from the bottle?
Zero for everything?
What about one where you put the oil into the engine and drain it out again a few days later without having run it?
The ppm numbers (on oil analysis I have had over the years) won’t be corrected for the sample duration – they are just straight ppm of that metal in the oil – but the analysis needs to take account of it. And same with oil added during the interval, which has the effect of refreshing the oil. For example this is my last report
The top line is the sample duration in hours. The short interval in 2019 was due to a cylinder repair and a re-hone.
Lead tends to track the sample age pretty well – it comes from burning 100LL. The rest varies.
You typically state the number of hrs and age of the sample that you send off; the analysis place applies the needed math.
I have a question about oil analysis.
If you change your oil at 50 hrs once and the next time at 25hrs, do you have to correct the results ? How do you make trends with variable oil change intervals ?
That’s fine.
I am tempted to have oil analysis done at the coming annual. I have never done it before.
As I have an engine monitor, I do keep a beady eye on everything as much as possible.
I have 1 slight anomaly in that when rich at Idle all temps CHT/EGT are similar but when lean at Idle cylinders 1&2 have noticeably higher EGT. When in the cruise they are all quite even again. It’s been like it since before she was mine.
I mentioned it to the mechanics but they didn’t think anything was a cause for concern.
Relevant threads here and here, among others.
The reason low compression doesn’t lose power is because everything happens so fast that the gases don’t have time to escape through whatever is leaking.
But you ignore this at your peril, because if e.g. a valve is not seating this is likely going to overheat pretty fast – see e.g. here – and when it fails properly you will definitely notice That event has been an eye opener for me, making me check low RPM running very very carefully, both immediately after startup (the best time to show up sticky valves) and before departure, check all EGTs one by one, and then again immediately after landing, again checking the EGTs at idle. IOW, poor compression does show up at low RPM – as you would expect.
For some reason Conti have very low airworthiness limits on their compressions, compared with Lyco. I don’t know why. It’s a much discussed topic. Some say it is poor manufacturing by Conti. On a Lyco IO540 you expect to see 77/80 or better for the whole TBO, although the exact number depends on whether the engine has just been run, etc.
The current corona virus discussion – which is obviously running all over the place, with much sanctimonious pontificating about whether human life is worth more than some engine – highlights that almost nobody does oil analysis, and thus is not going to know if there is corrosion… until their camshaft starts to relocate itself into the oil filter but hey that is not a problem because the plane will be sold by then 
Once you get behind (and have to pay for)a big bore continental you quickly either have a nervous breakdown or learn to almost ignore compression values.
I have been told by many that they are not easy to check reliably.
If I pull the prop through when cold, it is surprising how little compression there is. It’s marginally better just after landing but not like my old IO360 180hp.
However it always fires on the 1st rotation, seems to have all the power I need and more, flies happily at the performance the book predicts (allowing for manufacturer salesmanship),
and doesn’t move on the dipstick between oil changes. Go figure.
