Originally Posted by JCL

My wife insists on a manual transmission. Both our daughters drive manuals. I have owned lots of automatics (as many vehicles don't give you a choice).

While there is a chance of failure whenever maintenance is done, I don't equate an oil change with a transmission fluid change. If the oil change is done correctly (procedure, fluid spec, etc), there isn't a downside except cost, and we know that the cost is repaid many times over in extended engine life. That makes it a good decision to change the engine oil. On the other hand, failures of perfectly well functioning automatics after a fluid change are well documented. I have seen them. I am not referring to transmissions that were on their way out, or where the wrong fluid was used. I am referring to the replacement of perfectly good (albeit aged and dirty) fluid with new fluid, containing a high concentration of detergents, and involving a change procedure than can move sediment from where it wasn't hurting anything (except our individual senses of what is right, ie it shouldn't be there) to where it can hurt something (a valve body, a check valve, an actuator). Incidentally, I have noticed that almost every time someone changes old fluid, they comment on the junk in the pan, as if that is justification for the change. That is where the junk is supposed to be. It is what the pan is designed for, to hold it. But failures after fluid changes happen all too often. It is a risk, not a large risk IMO, but a measurable risk. If the fluid is worn out, burnt, oxidized, etc, then it is well worth changing it (except that if the clutches are burnt, it is probably too late). But if there is no appreciable extension of transmission life to be expected from a change of a dirty aged, but serviceable fluid, then that risk of changing it becomes relatively more significant, and it is likely better to just leave it all alone.

The best maintenance strategies do not try to keep components like new. They try to keep them functioning as required, balancing cost against benefit.

I am not sure my own maintenance intervals should matter much, as they are based on my vehicles, driving conditions, fuel quality, etc, but to satisfy curiousity.....it depends on the vehicle and the application. This may be a little long and detailed. Also, this is what works for me. I am not Mike Miller, and I am not guaranteeing the results for others.

I have recently changed oil on a Toyota and a Civic (family vehicles) at 5000 km. Good interval for those engines IMO, they hold about 3.5 litres. My BMW experience ranges from a 318 (1994, spec'd at 16,000 km interval) which didn't make enough heat on my drive cycle to keep condensation out of the valve cover. It formed a paste, commonly seen. I drove 30 km to work, and it was a small and efficient engine, ie not much waste heat available to boil off the condensation. Changing oil at double the spec'd interval worked, ie 6000-8000 km. My E53 was spec'd by BMW at around 24,000 km (15,000 miles, modified by CBS). No condensation issues, same driving route. It got hotter, I presume. It also used a newer spec oil than the 318. I went to 6000 km (myself at home) and 12,000 km (dealer). I pushed that up to 8,000 and 16,000 with good results. With the 325xi, used on more short trips, I went 6,000 and 12,000. BMW did low mileage changes at 12,000 km on that one, despite them not being required until around 24,000 km. Z4 mainly sat in the garage waiting for sunny days, so it got very low mileage changes as I consider sitting to be harder on a vehicle than driving. Flash forward to the 535 and X3. Newer generation engines, newer generation oil spec (API SM). Haven't done any intermediate changes. Computer last called for service at 26,000 and 28,000 km, but I didn't let them go that long, closer to 22,000 and 24,000. No more short trips, they always get hot, and neither used any oil to speak of (1 litre at 12,000 km on the 535 IIRC).

Engine oil on the BMWs is from the dealer, or Castrol Syntec 5w-30. I have used Mobil 1 for top up, and have no qualms using it instead of the Castrol. Those are the only three oils I use on the BMWs.

I only use OE filters, no will-fit filters, no matter who claims to make them.

No automatics in the fleet now, but I haven't changed fluid in any of my last 8 automatic transmission vehicles. Last transmission failure was on a 1966 Mustang in the late 70s, and that includes many years of towing with SUVs.

Brake fluid every 24 months, rigidly adhered to.

I would change coolant at 4 years, but haven't had to for some time. It used to be every 2 years, some time back.

Manual transmissions and differentials, I used to use 50,000 km (generally more frequently than called for) but would now go 100,000 or 150,000 km. If there was a chance of water ingress (fording creeks, etc) I have changed fluids immediately afterwards. The BMWs don't ford creeks, the Fords did.

Transfer case used to be the same as differentials. With the reset required on the x drive on our X3, I am inclined to leave it to 100,000 then have the dealer do it, as I don't have the tooling to reset it.

Other maintenance: Air filters based on driving conditions, ranging from every two years to every three months (extreme dust). Brakes inspected twice per year when the winter wheels are installed, caliper ears lubed with HT grease, slide pins kept dry. Hub bores cleaned when tires are changed. Tires never rotated on the BMWs, even though they are off. Door hinges and latches greased annually (amazing how many don't do this). Full inspection underneath (any signs of leaks, everything tight, CV boots, etc) has been annual, but was every six months years ago. Spark plugs were every 5000 km on the Toyota, a little longer on the Honda, and I haven't touched them on any of the recent BMWs. I would think 100,000 or 150,000 km, but I don't have experience to base that on. I would inspect them at 100,000 km.

As to data, my own maintenance schedules start from the manufacturer's recommendations, and are modified from there based on driving conditions, my own historical results, ongoing monitoring and inspection, and my own experience and judgement. That is based on being a mechanic a long time ago (when we used fixed 3000 mile oil change intervals), and then several decades as a professional mechanical engineer involved in heavy mobile equipment maintenance. That maintenance involved oil and fluid sampling, guaranteed maintenance contracts over 60,000 operating hours, guaranteed fleet mechanical availabilities, lots of predictive techniques like vibration and performance monitoring, etc.

Told you it would be long. Sorry about that.

Jeff