Price is probably the only real bad thing. But it's a good idea to use syn. Cleaner and won't break down as quick. I'm probably going to do M1 or something alike. I know in my 335i, hell it needs the Syn ps fluid. Sheit turned dark brown by 40k. But it take the fancy german stuff in a metal can, no options there.
Edit: Dex VI should work where any prevoius Dex II or Dex III is called for. Backwards compatible. And I believe this is GM's most updated ATF, which is syn or part syn. I'm probably going to go that route. When our E39's were made, Dex III was current. VI wasn't even a thought back then. So yes, something like the M1 ATF or Dex VI Castrol would be a good thing at lest for keeping it clean and high temps.
Edit 2: Some good GM info regarding the Dex VI as ps fluid in a dex III system. GM started using it in 2012. They seem to be worried about a seal squeak when converting sytems over. Not sure if that's an issue with a BMW b/c GM makes every plastic rubber out of road trash. But good read none the less.
Bob is the oil guy forums have good info. I'm going either Dec VI or M1. I wouldn't mind going to Pentois CH11S like my 335i, but that maybe to much of a change. I'm going to disconnect the resv return hose and make sure I get a 100 Flush.
Edit 3: looks like Dex III has (had) a certain visc and M1 ATF is for that application. Mobil makes a specific ATF for Dex VI appliations. So Dex III and Dex VI are different in tems of visc. Gm is ok with you running the newer thinner better fluid in the older transmissions. But M1 ATF is NOT for Dex VI applications. BMW's CHF 11S is basically their own european version of going to Dex VI p/s fluid. But even thinner. So from thick to thin; Dex III (M1 ATF), Dex VI, CHF11S.
40*C Visc, 100*C Visc:
Mobil 1 ATF (Dex III): 36.3, 7.4
Mobil Dex VI: 29.5, 5.83 [Castrol Dex VI is 30.0, 6.1] Dex VI is a "SYN BLEND" by spec I think.
CHF 11S: 18.7, 6.0
I'm probly one of the only people out there that wants to switch from cheap ATF to expensive CHF 11S!! But I'm not sure how that's going to work. I know you can't mix them... but even after a flush I question the seals and that a extreme Viscocity change. I'm going to play it safe and go middle of the road with Dex VI. See how that does first. Then "maybe" flush out to CHF 11S.
DD: 2003 530i 5spd manual sport package. Conti DWS's, Llumar ATR, White LED lighting. TLC on-going.
Garage queen: '07 335i Sedan, base pkg, 6spd Manual, Procede 3 V5 w/NLS, staggered 18" 313's w/Conti DWS's, OS Giken TCD LSD, VAC Street Diff Cover, Akebono Euro Pads, CDV Removed, Black BMW Grilles, Interior & Exterior white LED lighting, LUX Angel Eyes, Debadged, Llumar ATR Tint.
Last edited by FLE39; 11-19-2012 at 08:52 PM.