I am interested in reading the SAE papers about the cam galling at 1800 ppm and 100% probability at 2000 ppm. 

The suggested zinc level for our race engines is 2000 ppm and for new cam/lifter break-in is 2500 ppm.  Redline racing oil 50 wt contains 2310 ppm zink. 

First, yes, 15W-40 diesel will be fine. Not as great in cold starts, but off-the-shelf good as far as additives are concerned.

As for the ZDDP level, I hate to be the bearer of bad news, but 1800 ppm of zinc is the entrance to the trouble zone. Several SAE papers are written about a high probability of cam galling at 1800 ppm and 100% probability at 2000 ppm. That is why I suggest people not play chemist. To be safe, you want to keep it under 1600. I really doubt that you will see any difference between the standard CI-4 with about 1300 ppm and 1600.

I've analyzed over 5000 samples, but none with that high a ZDDP level, so on that I have to rely on the SAE studies and publications, but it is a pretty reputable source.   

We sent off oil samples to be tested to get real information on oils we were using in our street engines focusing on the zinc levels.  Our high valve spring pressures need a zinc level of 1800 ppm.  The common oils tested was Castrol GTX 20w50, Valvoline VR1 20w50 and Amsoil ARO 20w50. We tested others but more for racing applications not mentioning here. 
Castrol tested at 1232 ppm
VR1 tested at 1390
Amsoil tested at 1175
We took a 5.5 qt jug of Castrol GTX and added a 4 oz bottle of ZDDPlus and sent that off to test.  It came back at 2237 ppm zinc. Plenty for our engines. With that you know the bottle of ZDDPlus will add enough zinc to any of the oils listed here. CamShield is another additive which does the same thing at about half the cost per oil change. Their website is full of information if you're inclined.  Zinc levels were reduced to lessen problems with catalytic converters in today's vehicles. Haven't tested any diesel grade oils, will have to send one off to see how it compares to these oils. 

Thanks guys, this is exactly what I needed, great info here!  Forgot to mention it's a manual.  I'm in MD so looks like I should use a 10W-40 or 20W-50 non-synthetic w/zddp additive?  Would a 15W-40 diesel dino motor oil be good as well? And Bosch 3330 or Wix 51348 filter.

Donaldson is the world's premium filter manufacturer. You don't see them much in retail stores because that is more of a price and cut-throat operation, but you can find them on line and in some places more oriented towards heavy equipment. I am a distributor for them, and have about 55,000 filters in stock, ranging from $3 to $800 each.

Bypass valves are critical for most engines. On my Corvair it is not necessary as the adapter has one. I still use a filter with a bypass valve, but one that opens just slightly higher than the one in the engine, so it that were to fail, the one in the filter would open and let oil go through to the bearings.

Many brands have absurdly cheap valves that stick, either closed or open. If stuck closed, you could have oil starvation in the cold, with thick oil, or when the filter clogs with dirt or soot. If stuck open, you are not filtering.

The pressure the valve should open depends on the engine design and the pump. Some engines use a bypass pressure around 36 psi., knowing their pump will easily push that amount up, and that they want clean oil to the bearings. Most cars are in the 8 to 15 psi range. The manufacturer puts the priority more on having oil, than having it clean. 

Richard: what are Donaldson numbers? When you say a particular filter brand number crosses to a Donaldson number, does that mean the brand matches the spec of the Donaldson number, or are you saying the Donaldson (assuming for the moment it is a brand) is a good filter?
Another question: I have seen/heard about filters with and without bypass valves. I think I remember negative comments about a brand or brands where the bypass valve was weak or failed. Does a Mini (normal summertime driving) require a bypass valve type filter?

That is an interesting list, I don't have time right now to look bak to original specs and see what is what, but they are not all equivalents. I looked quickly at a few, and the 4 that I checked crossed to 3 different Donaldson numbers.

The purloator crossed to an Opel/Deutz filter (P551251) that I don't have all the specs for (bypass pressures being important).
The Valvoline number crosses to P502048. Be careful with this one, since, if all data is correct, the rubber seal may not seal well, as there is only a 2 mm overlap from all the rest. But it also is a M22 thread, so won't fit a ¾-16 thread.
Wix and Bosch crossed to Donaldson P550335, which is an excellent filter
Fram crossed to Donaldson P550400, which is also an excellent filter, with much higher flow and holding capacity than the P550335, as it is 1½" longer, but essentially equal in other aspects. Bypass valve starts to open at 7 psi and is completely open at 10 psi.
Bypass valve is particularly important for anyone not running a multigrade oil, and for people using 20W-50 where temperatures are below 50ºF in the morning. 

The automatics are different as they have to handle 175 psi pressure in reverse.

Canadian Tire is a retail chain in Canada that has in each store a large auto parts section that includes a well stocked parts counter.
17-1883-4 is the stock number for their Motomaster house brand spin-on oil filter for Mini engines.

Hi kd, 
this thread is all about oil and oil filters.

Canadian tire 17-1883-4

Here's a few of the 40 or so filters that will work:
AC-Delco 93156323
Mobil M1-102
Unipart GFE148
Walmart ST3614
Bosch 3330
Fram PH3600
Wix 51348
Valvoline VO22
Puralator L10028
Quaker State (with similar number as the Fram)

While will agree with the recommendation against Fram filters, I'm afraid you are a little out of touch with lubrication. I have been studying and teaching lubrication for more than 20 years, day in and day out. I have published close to 200 articles on maintenance, many on lubrication.

20W-50 has very few uses. The biggest argument for it in the mini is for the manual transmission, since it has the viscosity of SAE 90 transmission oil when hot. But 10W-40 works fine in a Mini transmission that has been properly rebuilt, a friend is using it in the rebuilt box I imported for him. And the Mini engine works better with 10W-40, better lubrication and better cooling.

sir Bill, really you are using 40w motor oil???? gosh that is too thin for our classic mini engines...i bet you hear metal to metal clanking on your minis.  the purpose of the 20/50w viscosity is to give cushion upon metal to metal friction.   40w motor oil is really risky to use if you ask me.

Stick with 20/50w Castrol motor oil and 1 small can of restore.... mix them together and pour  into you engine reservoir.  the restore (engine restorer) is for added protection only,, it's optional.

Avoid using fram oil filter, they are junk.......i use Mahle oil filter(same used by formula 0ne racers)
.....well you can also use Unipart oil filter(purists love this filter, i don't)

If the engine is in good shape, 10W-40 is much better for circulation and cooling. I rebuilt mine completely, so went to 10W-40, running 60 psi until it warms up, then 40psi, so anything thicker would just take away power and lubricate less. Normally this is all you would look at to determine if you needed anything thicker, but with a Mini, you need to consider wear in the transmission, but 20W-50 will not synchronize as well as 10W-40, especially in the cold. Worn shafts would make more noise with the thinner oil. 

Look for an oil that is formulated for diesels (CI-4 or CJ-4) for the extra ZDDP for the sliding transmission and valve train parts. I do not recommend using additives, as you are probably not a chemist, and too much is worse than too little. Any additives you put in, without knowing the additives in the oil to begin with, will be unbalanced. There are good oils already formulated.

And stay away from single grade oils. They cause much more wear every time you start the engine, cause more resistance against the rings when the piston is going down, and are basically obsolete. No one has put any technology into them. The additive treat in them is very weak, allowing more wear and carbon build up. 

If it an automatic, most important is that it meets the JASO MA2 motorcycle oil spec, as that will make the clutches last several times longer. And 20W-50 does not displace very well on the clutch plates in the Automatic.

Oil can be purchased at regular auto parts stores...depending upon where you live. Correct weight? Add to mur's question, where do you live? North pole or Panama; i.e., what type of weather to you normally drive in? I live near Seattle, WA, and run 20W-50 all the time in my manual transmission cars.
The Mini spin on filters (assuming manual transmission) are interchangeable. Short or long. Both work. With the short filter, you'll need about 4.5 (U.S.) quarts. Five if you use the longer filter.
Look for oil that still contains ZDDP, like Valvoline VR1, or buy an additive to compensate for the lack of ZDDP in almost all motor oil now sold in U.S. auto part stores.

I have always used Straight 40 W.. in ALL my classic minis for the past 20 plus years..  Advance auto parts.. Auto Zone.. or even Wally World has it. in stock.. later b c

Does this car have a manual transmission or an automatic?

Can someone recommend where I can purchase motor/trans oil for my 1990 998 Mini?  What's the correct weight, 20W-50?  Are the oil filters shared between the different size motors?  Is there a page that lists capacities, etc?  Sorry, new owner here, thanks for your patience!