We put two filters on most captive(vehicles) and as an option on other engines.  The outer filter gets changed 3 times before the inner.  The customer can still chose oil bath as a special option but it is rare.

In my older Donaldson guide and on this site posted earlier http://www.fallsfti.com/prod_oilbath.htm the efficiently
runs about 95% not 89%.  You are correct that oil filters do worse as time goes on and paper better.  However only the best paper filters reach 95 increasing to 99.5 percent as they load up.  Most of the current cars and trucks do have these filters, normally the square or rectangular shaped ones, but most of your circular cleaners(they don't seal as well either) found on most carburetor models didn't come near that.  Smaller paper filters found on lawn equipment do even worse.  Now that is not all that bad let me remind all of us.  The original listers had an oil felt filter which undoubtedly did worse yet they ran tens of thousands of hours without
problems.  The paper filter set up shown by gorge's cd is perfectly adequate to our needs that our stationary engines which have cleaner air than probably any other application other than aircraft. Car Quest makes these filters to the original manufactures specifications and who exactly knows what that specification is.

Lets for all practical purposes say on average that ALL TYPES of filters come close to the same overall average performance of filtering.  Now what becomes apparent is the way it is implemented.

The main reason that paper filters have become common is that many people espouse the phrase "time is money".  A paper filter, even if it cost 10 times more to replace than the oil, is money well spent because in many places labor is expensive and down time is even more(a piece of heavy equipment can make a company 1000's of dollars an hour).  The other reason is there is always someone screwing up and destroying things.   People can over fill an oil cup and can let it go exceedingly long periods of time without changing them.  Neither of these reasons really apply to us, we are hobbyist for the most part, care and are well educated about our equipment, and are not liable to abuse or neglect the engine cause we paid for it as opposed to the idiots that don't care what happens to their employee rs equipment.

I never have enough money, for there is always someplace around my farm or other hobbies to bury it in. I do have time.  The convenience of a paper filter doesn't equate to the cost of the oil for an oil bath filter for me.  Half a quart of oil is 50 cents.  Enough oil for quite a few changes is easy to store on one of my shelves and saves me gas into town.  I feel that I am competent enough to fill oil to a line in the bowl and that my wife is as well (I am lucky to have a wife that likes to get her hands dirty) therefore eliminating the runaway issue even if there was one on this engine to begin with.  If I buy paper filters and store on the shelf, invariably a mouse decides that it would be a great place to make a nest, destroying the filter.  I have even seen these buggers make nest in paper filters on an engine that has set idle for a few months.  Oil bath filters are still easy to clean, dump the oil and wipe out with a rag.  Washing the whole unit with solvent is not necessary because the clean oil mist will condense and fall back to the cup with the dirt, If washed with solvent the unit will allow dirt to flow by for the first few min until the unit is coated with oil again.  Washing them out is only recommended where someone has let the service interval exceed and become excessively plugged with dirt. 

I must admit that sizing of 4 cylinder models didn't come to mind when I wrote the first post, and at first I agreed upon sizing to one cylinder of a 4 instead of all four.  I did a little research and realized something else, RPM matters as much as  cylinder size.
Most fours in tractors operate about 1600 to 2000 rpm.  Here is an equation to use from above mentioned site.  Cubic Inch Displacement x Max. R.P.M. x .00029.     According to this an International Cub would have a filter of similar size as would a John Deere LA both about 27-29 cfm.  a 6-1 is around 16cfm,  My WAG is the slightly larger CFM would allow for the single cylinder to breath enough in one gulp but I need to do more research.  I truly believe sizing off of one cylinder of a multi cylinder engine without factoring in rpm would be way to big.

As for the K&N.  I know it protects your race cars, but how many races do you get before having to rebuild?(not saying that you have to rebuild because of air problems)  Most sprint races I have been to there is little dust, as they wet the track just enough to make it pack.  There is mud, which is quite a large particle to any filter.  I have a friend that uses K&N on his sprint car, he says that it does make more horsepower and as long as it keep the major chunks out that would prevent him from finishing a race he will continue to use them, rightfully so in my opinion(He gets about 10 race nights out of an engine before he goes through them).  Racers care about winning, some drag racers use no air filters at all and rebuild after every run.  I am not saying it is a bad filter, just that it isn't great simply because it is race technology.  Most things on the race track have no place in an engine used in every day life.  My only real stink about the K&N is the cost.  The initial cost is pretty salty and then you have to buy the cleaners and the special oil to re-oil.  I have seen failures with these filters but I would say that they were more likely due to applying the oil in an nonuniform manner missing spots and mechanical damage, not because of the filter design. 

You are correct hotater, the cam is supposed to be radiused with concave lifter or else edge loading is extreamly high.  Maybe the indians could copy the lifters but lack the machines for the cams?  Are the origninals radius?  Automotive lifters are normally 30-40" circle ground and larger diesels are 80-120 inch ground.  Grinding the lifter flat might slow down the rotation somewhat but it would surely reduce edge loading and all the wear problems related to this. 

Thanks,
I was looking at 3208 rings, they are the same width but are also deeper. (.184)

Can anyone take a look at there 6/1 rings (Compression) and mike the thickness and width (metric prefered).  I found a possible high quality replacement, the bore is 4.5" 114.3mm lets see if the other measurements add up.