“Would you open an engine if you see stuff in the oil filter but the strainer is clean?”
When the oil filter is fitted, the screen must be removed – as you’re no longer checking it, it could get blocked. I’m told this has happened and caused an engine failure.
The O200 screen is much easier to access on the Jodel DR1050 than on the Bolkow Junior.
So the oil filter goes where the screen was?
Savvy (Mike Busch, et. al.) have published a short newsletter about this issue.
In the engines I know about (Continental TSIO-360, Lycoming IO-360) there is both a spin-on filter and a screen and both should be inspected at least annually, but looking at oil filter adapters on Aircraft Spruce it looks like they indeed replace the oil screen, at least in some cases.
The O200 screen is cut off and the housing replaced. There is provision for fitting an oil cooler, covered with a blanking plate. That is removed and the filter adapter replaces it.
I don’t know about other engines.
I didn’t realise the one which failed was the one currently sold under the Tempest name (the original STC holder has long vanished). Normally Tempest make good stuff.
The last paragraph is highly relevant. It’s a crap design since the gasket is more or less assured of losing the bolt preload, over time. This is why there is no gasket under engine cylinders; you would have no chance of maintaining the bolt/stud preload if you had one.
The C90 in our Vagabond has no oil filter, just the screen which would only catch things big enough to have a part number on them. We change the oil every 25 hours.
There are options to fit a filter with an adaptor, although in our installation I believe the options are reduced because of a lack of clearance to the firewall. The saving from going to 50hr oil changes would likely be offset by the cost of filters.
I have to say I can’t think of any reason why you wouldn’t for a oil filter apart from them failing off which they really shouldn’t do.
I also can’t think of any reason why a manufacturer would sell an engine without an oil filter fitted.
A filter is about 15 quid. Not really significant; one burns several times that money on oil during the same service interval.
It’s interesting to discuss what purpose it serves IF the screen really does trap all particles big enough to damage the engine. However I strongly suspect that the screen does not trap all such particles. The screen in my IO540 has ~1mm holes in it, and bits of steel of that size will do a lot of damage, to the accessory gearbox for example.
Another thing is that general “muck” in the oil is very capable of blocking up the crankshaft oil passages, with an obvious outcome, but before that outcome you may get a loss of propeller governor function – probably what this guy had although he never posted that.
And another is oil seal function, which does rely on the oil being clean. If there are sharp bits in it, that will shag the seals. There are very few seals in a Lyco engine but the front seal is hard enough to replace.
Perhaps the mechanically best solution is where you have a small adaptor and you have two hoses running to a filter mounted separately. Then you have the risk of two hoses developing a leak, however. And the stress on the hoses will depend a lot on whether the filter is mounted on the engine or on the airframe 
Peter wrote:
A filter is about 15 quid. Not really significant; one burns several times that money on oil during the same service interval.
£25 plus VAT and delivery as far as I can see, which is approximately the same as the oil necessary to do the 25hr change that it would negate.
Not sure about burning that amount of oil between changes. Neither the Vagabond nor the TB10 burn any significant amount of oil. The only time they lose oil is when someone overfills them. The Vagabond certainly burns well under a quart between changes, but they are every 25hrs.
To fit a remote filter on the bulkhead is a fair job on a Vagabond, and not one we have an appetite for at present.
My plane has an (I believe unapproved) firewall mounted filter, and it really does make life easier for filter changes. I replaced all the hoses to filter and oil cooler and they still make me nervous, but it may be irrational because the oil cooler itself is fragile and isn’t so much a concern to most people.
