Great Ozzie wrote:AKar wrote:For a much-flown airplane, where good engine management is employed, I personally wouldn't see a problem in extending the oil change interval to 100...200 hours, instead of common 50.
Personally I would see a problem. A big problem. The only time I have *ever* heard of a 200hour oil change was when the FAA granted an STC for an 200hr change for Mobil AV1 -- but that stuff was yanked from the market years ago.
Okay, "no problem" is perhaps too light attitude
, but let us get a bit behind the point. With that hypothetical high-time airplane btw, I mean one accumulating
really high time when compared to typical GA, as in, say 8 hours a day for example, for some time span. Say, during that operation we wanted to change the oil every two weeks. That is, approximately every 112 hours, putting us into aforementioned range.. or maybe three weeks, which would mean 168 hours. The time the oil sits in the engine won't get very high if we for some reason wanted to skip the 50-hour check and go straight to the 100 for instance. For sake of it, let this hypothetical operator go to maximum insult, and do the oil change every 3 weeks / 168 hours. (To underline,
I would not suggest this! Only for discussion, so to say.)
What becomes a factor is that these engines do consume oil. It could be something like one litre/quart in ten hours, as one rule of thumb is. So, it would not be unreasonable to expect the operator to top the oil, just say, every 30 hours.
So, while it should be taken with some grain of salt, being just an unchecked back-of-the-envelope estimation, the operator in our example would add three liters of oil, five times during that three weeks, essentially renewing 25 % of oil every 30 hours. Assuming an oil capacity of, say, 12 liters, we can quickly approximate that by the last top-up being done at 150 hours, only one fourth of the oil or so, would be original, whereas over half of it would be newer than 60 hours old. Actually, when finally getting drained some 18 hours later, more oil from the last top-up comes out than what remains of the original. And even that original has been in for just three weeks, with barely time to cool. Now, of course, it doesn't work like with fruits on stand in reality, but just to get the general idea.
Now, compare this with how some (too many!) airplanes are, or at least have been, operated. They accumulate less than 50 hours a year, and the 4-month recommendation is often ignored. To compare with another maximum insult, take a case where the airplane is flown maybe 30...35 hours during the summer, and only in very few occasions during the winter, once a month maybe. The airplane then gets its typical annual "spring maintenance" where the oil, most of which is year old, finally gets changed.
While, again, not something I'd suggest, the
problems I'd expect from the first kind of use could very well be less than those one may run into when dealing with these low-time airplanes of the second example. Obviously, one tends to run into entirely different kinds of issues if such high utilization was attempted with most GA types...as someone said a while back, making the exact oil change interval the least of his problems!
And once more, to underline my personal view, I consider the oil changes the cheapest kind of preventive maintenance. I see no reason to push those, especially in cases of low utilization. Those suggested in many instances in automotive world are almost ridiculous; even there I'd recommend changing oils at least annually.
Great Ozzie wrote:Read "Lie #3" from The Ten Biggest Lies About Piston Aircraft Engines.
Single Vis oils still recommended by engine re-builders such as Ram Aircraft, Penn Yan, etc.
Yes, though I wonder where the idea of multi-grades being "better lubricants" comes from. That's hardly a reason to go multi-grade. The entire reason of multi-grade oils is to allow a wider operating temperature range, especially extending the bottom range. As this topic is about cold weather flying, around here at least where we can have - and do have - large temperature variations during winters, an attempt to mess around with classic single grade oils while allowing for realistic storage and usage conditions for an average (actively flying) GA, is often like looking for trouble. They just are for different conditions. I quickly couldn't gather any experiences with relatively new AeroShell W80/W80 Plus, which appear to be rather well compatible with local conditions for most days.
When it comes to the rust issues, I'd expect bigger factors being in play than simple single vs. multi-grade factor, including differences in between oils+additives themselves unrelated to the grades. It is correct to note that the times of sitting are when the corrosion issues rise. Should the airplane be parked for extended periods, then the proper storage and preservation methods come a big factor. IMO, if an airplane is flown only a little during the winter period, so that one even needs to start thinking about single grades for maybe enhanced rust protection, or any other differences in this aspect supposedly caused by oil grades alone, I'd suggest treating the airplane like a motor cycle: park it in proper season storage for good instead of throwing dices with rust issues!
BTW, certain preservatives / corrosion inhibitors are, to my recollection, compatible only with straight mineral oils, which could be a consideration.
From the above, at least Ram Aircraft appears to recommend single
or multi grade oils as conditions require. They instead suggest that their experience is better with mineral oils in comparison to semi-synthetic ones (I quickly can't recollect any common full synthetic aviation piston oil). This is an interesting observation, and I've not heard of similar experience around here, except some religious views that go both ways. Could as well dig something up from the literature!
Edit: Yes, at least in 2012, Continental's calendar-based recommendation was six months in their "Scheduled maintenance" flyers. Hours are 25 (screens), 50 (small filters), and 100 (large filters). FWIW, Seneca V's (TSIO-360-RB) maintenance manual, page revised Jul 1/10, recommends changing the oil every 50 hours or 4 months.
-Esa