I changed all 3 holes of mine to Mobil 20-50 synthetic and its really quiet and smooth shifting

 

Use the amsoil 75w140 SVG in your trans and the Amsoil ATF or 10w40 MCF in your primary.

 

Next time try the mobil1 75w140 gear oil in your trans. You will like it even better.

 

You will like any of the synthetic gear oils that have been mentioned Mobil 1, Amsoil and Red Line. I am now using Red Line Shockproof Heavy in my tranny (which is a 90W-250) and like it better than Mobil 1 which I liked a lot better than Syn 3. You won't want to use the same stuff in the primary though - you can use anything from HD Formula + to ATF to motorcycle oil. Do a search on this site and you will get preferrences coming out your ears.

To answer the OP, I've never experienced a problem changing fluid brands in any vehicle.

 

Red Line Shockproof in the trans. Synthetic oil in my engine. The primary 20-50 synthetic or Dino Oil.

 

With Amsoil 20/50 in the primary, don't expect the clunk to go away when it's warmed up fully. It clunks like it's supposed to; I just trust the Amsoil more as a wear preventative. You may also hear the chain slap a bit when upshifting; clutch engagement is magnificent.

 

I would like to see to do Wash DC driving with that oil very long. Hey come to rolling thunder and tell us how your oil is after that ride.

 

So a Mobil 1 solution would be:

Mobil 1 20w50w Syn in Engine Oil
Mobil 1 20w50w Syn in Primary
Mobil 1 75w140 Syn Gear Oil in Tranny ????

 

I'm a little mixed up and hopefully you guys can help. I changed my fluids the other day in my new to me ride. I put Mobile 1 Synthetic in the engine, (20W-50), and Redline 75-140 synthetic gear oil in the trans, (even though I read on the bottle it says not to use it in manual transmissions after the fact), and I'm going to do the primary next.
What part of the engine does the clunking that forces people to try a different oil and then seem to think the clunk is reduced? Is it the primary or the trans that clunks and sounds gross?

 

Any oil of the same API rating can be mixed and used. Oil is stupid. It does not know who made it. This goes for dino and syn mixing also.

Not mixing oil is from the 50's when detergent oil was introduced. You were not to use it in a car that had the old non detergent oil as the new detergent oil would loosen up all the sludge build up and might clog things up. There was a shade tree mechanic way of changing to the new oil by draining the non detergent oil, fill the crankcase with kerosene and let it idle for a hour or until the temp gauge/light came on. Drain and put the good stuff in.

I did that to a 57 Plymouth 9 passenger station wagon and it worked.