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Synthetic vs conventional oil myths?! M42

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    Synthetic vs conventional oil myths?! M42

    Hi all - I just bought a 1991 318is and it appears to have an oil leak. I'll dig into the leaking problem further but upon asking the previous owner which type of oil he was using, he told me it was synthetic (Castrol if I'm not mistaken).

    I know there's an old myth that once a car has had synthetic, you cannot switch back to conventional. My questions are:

    1 - is that true??
    2 - if true, what is your preferred synthetic oil for an M42 with 114K original miles? I have zero service history.
    3 - if false, what is your preferred conventional oil for the M42?

    Thanks for any and all help!

    #2
    You said it, it's an old myth.

    There are a variety of threads in the section discussing oil choice which you should read. As to my M42, I like to use Castrol synthetic 20w-50, but will normally buy whatever heavyweight synthetic that's on sale.

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      #3
      Thank you roguetoaster! I'll check out the previous threads. I also prefer Castrol but am a sucker for a good deal.

      Comment


        #4
        I run Mobil 1 15W-50 synthetic. It is the proper oil weight for this engine. The M42's flat tappets require a very good oil with high ZDDP content, and Mobil 1 15W-50 has the highest ZDDP levels of any readily available oil. If you look in the owner's manual, the specified oil is 20W-50, and it looks like you live somewhere with cold winters, so maybe Mobil 1 10W-40 would be better this time of year.

        Metric Mechanic did a huge oil reliability test and basically, Castrol 10W-60 TWS oil was by FAR the best oil that they had seen, with Mobil 1 15W-50 being second-best. The Castrol stuff is $20/quart and it leaks like crazy from paper gaskets, so I stopped using it and am back with Mobil 1 again.

        As for your oil leak, I am willing to bet that it is the oil filter housing. If the whole front of the engine and oil pan are soaked with oil, it is most likely the filter housing o-rings. They are notorious for this. You can make the repair in a few hours if you are halfway decent at wrenching. You'll need the o-rings and a new filter housing gasket. Make sure that everything is IMMACULATELY clean on all surfaces before putting it back together, and take great care not to let any crap fall in to the circular hole at the top (which the o-rings seal) because this is the clean oil inlet to the engine. Do not let any gasket sealer or RTV get in here either, if you decide to use a gasket sealer on the paper gasket. You can rub a tiny bit of clean oil on the o-ring to make it slide in easier and not bind up.

        Good luck!

        Transaction Feedback: LINK

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          #5
          bmwman91 - VERY VERY HELPFUL!

          I'm not sure where the leak is originating as I don't have the car in my possession yet. It was purchased from a guy down south and it's mid-transit right now headed out here. I had a PPI done, however, and the tech noted some oil leakage. He suspected it was an oil pan gasket and a front crank seal (I'm paraphrasing). I haven't had a chance to get up under it yet and take a close look at where it's gooping from but it looks like a substantial oil leak. Since the previous owner said he was using Castrol...and Castrol can cause them to leak...perhaps that's what's happening. I'll use your comments and warnings upon my diagnosis and ensuing repair. I'll see if I can attach some photos of the leak in question.

          Comment


            #6
            The mobil 1 15w-50 is great because they have at wal mart, they have it in gallon and qt sizes:up: how much oil is it leaking?
            -Christian

            '02 ///M3 CarbonSchwartz 6MT daily beast
            08/91 Mtechnic II 325IC alpine/lotus
            318iS, slow build/garage queen...
            '37 Chevy pickup, the über project
            Originally posted by roguetoaster
            Be sure to remind them that the M42 is one of the best engines ever made, but be sure to not mention where it actually falls on that list.

            Comment


              #7
              Der E30 - I'm not sure "how much" oil is leaking to be truthful. The mechanic that did the PPI just told me it was leaking and needs to be fixed. He didn't say at what rate but made sure to let me know it should be fixed asap (I don't disagree).

              Comment


                #8
                Yeah, based on those pictures, I would say that your oil filter housing is leaking. It always LOOKS like the oil pan and crank seals based on what you can see, but what is almost always happening is that the o-ring is shot so oil runs down the front of the engine behind the damper wheel. Synthetic oil in particular has a propensity to run and spread out, making it hard to diagnose the source of the leak.

                Here's the diagram of the assembly. BTW, if you didn't know about RealOEM, get to know it because it will be your best friend (because this is an old German car, and you WILL be working on it lol).


                The culprit is typically part #8. Sometimes it is also part #9. However, #8 is almost always responsible for ruined driveways. You have to replace both to get the job done, so I guess it doesn't matter which one is leaking but my money is on the o-ring. Replacement is fairly easy. Disconnect the battery (do NOT skip this), remove the air filter box/air meter/intake boot (all as one unit), disconnect and remove the alternator, remove the alternator bracket. You then have nice clear access to the filter housing bolts.

                Anyway, here's the thread I made on the topic last year.


                Post 18 has some details about what I used to seal the gasket. If you do decide to tackle this feel free to ask for tips. It is a straightforward job, and as with ALL repairs involving gaskets, immaculate cleanliness of the sealing surfaces is required if you don't want to have to do the repair again 3 months later.

                Transaction Feedback: LINK

                Comment


                  #9
                  Thank you so much!! The car arrives on Thursday so I'll probably dig into this over the weekend and will update.


                  Sent from my iPhone using Tapatalk

                  Comment


                    #10
                    Originally posted by bmwman91 View Post
                    Yeah, based on those pictures, I would say that your oil filter housing is leaking. It always LOOKS like the oil pan and crank seals based on what you can see, but what is almost always happening is that the o-ring is shot so oil runs down the front of the engine behind the damper wheel. Synthetic oil in particular has a propensity to run and spread out, making it hard to diagnose the source of the leak.

                    Here's the diagram of the assembly. BTW, if you didn't know about RealOEM, get to know it because it will be your best friend (because this is an old German car, and you WILL be working on it lol).


                    The culprit is typically part #8. Sometimes it is also part #9. However, #8 is almost always responsible for ruined driveways. You have to replace both to get the job done, so I guess it doesn't matter which one is leaking but my money is on the o-ring. Replacement is fairly easy. Disconnect the battery (do NOT skip this), remove the air filter box/air meter/intake boot (all as one unit), disconnect and remove the alternator, remove the alternator bracket. You then have nice clear access to the filter housing bolts.

                    Anyway, here's the thread I made on the topic last year.


                    Post 18 has some details about what I used to seal the gasket. If you do decide to tackle this feel free to ask for tips. It is a straightforward job, and as with ALL repairs involving gaskets, immaculate cleanliness of the sealing surfaces is required if you don't want to have to do the repair again 3 months later.
                    Great post, thanks for the update.I believe I have this issue as there seems to be oil collecting int the little groove of the block behind the filter housing sort of under the intake manifold on my car. It also seeps drown the front of the timing cover to the oil pan where i watched it flow between the two front most driver side oil pan bolts, seen here.




                    Will be giving these two gaskets a try to see if that is the source of the leak.
                    318iS Track Rat :nice: www.drive4corners.com
                    '86 325iX 3.1 Stroker Turbo '86 S38B36 325

                    No one makes this car anymore. The government won't allow them, normal people won't buy them. So it's up to us: the freaks, the weirdos, the informed. To buy them, to appreciate them, and most importantly, to drive them.

                    Comment


                      #11
                      Man, that is a nice clean looking M42 lol.

                      Occasionally, the pressure switch will leak too. You might want to look into replacing the crush washer, or using a little RTV at the flange of the switch.

                      Transaction Feedback: LINK

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                        #12
                        I'm not familiar with the "pressure switch" - can you describe to me where that is located or perhaps show me?

                        Comment


                          #13
                          Oil leak? What oil leak? That looks better than mine! Mine consumes maybe a qt every 5k miles.
                          -Christian

                          '02 ///M3 CarbonSchwartz 6MT daily beast
                          08/91 Mtechnic II 325IC alpine/lotus
                          318iS, slow build/garage queen...
                          '37 Chevy pickup, the über project
                          Originally posted by roguetoaster
                          Be sure to remind them that the M42 is one of the best engines ever made, but be sure to not mention where it actually falls on that list.

                          Comment


                            #14
                            Originally posted by bmwman91 View Post
                            Man, that is a nice clean looking M42 lol.

                            Occasionally, the pressure switch will leak too. You might want to look into replacing the crush washer, or using a little RTV at the flange of the switch.
                            If the crush washer doesn't fix it consider replacing the switch itself as I've had two develop leaks through the body of the switch itself and leak out at the connector!

                            Originally posted by Nürburgring View Post
                            I'm not familiar with the "pressure switch" - can you describe to me where that is located or perhaps show me?
                            It's #12 in this diagram http://www.realoem.com/bmw/enUS/show...diagId=11_4331

                            In case it's unclear the switch is aft of the oil filter canister.

                            Comment


                              #15
                              ohhh that makes sense. Got it!

                              So to do the job, I'm going to be replacing part #s 8 and 9...and possibly 12 if 8 and 9 didn't fix it...and obviously all the seals that come with a new oil filter. anything else?

                              Also, do I need to use RTV as noted above with the replacement parts...or is that "above and beyond" in your experience?

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