When you run 20w 50 you are running 20 wt oil through your tranny and diff. You are at risk for failure for those components. Then the little bits that get past the screen go directly to the pump and then to the filter. Most transmissions of the era are running 90 wt so the straight 50 wt is somewhat of a compromise. Running hipo 1380 with over 100 hp needs the extra protection of st 50wt. Not my opinion just a few facts

Yet another vote for Brad Penn 20w-50.

I am hoping to get strait 50w castrol. It is usually warm here in san jose so no need for multi wt. the fact that the p/u pipe is right below the gear stack should require st 50w. I really can't think of any other car that pulls engine iol from the gearbox like the mini does.

I've been using Kendall GT-1 20-50 and been happy with it.  

Brad Penn  20-50

X 2 Peter. The zddp is more necessary in fresh re builds and i have not seen any more wear in engines with miles on them using straight 20w - 50, i use Castrol or Valvoline usually whichever is on sale.

Has anybody used zinc additive?  

Bradd Penn 20w-50.  I just ordered a case from lnengineering.com at a great price.  I've used in air-cooled cars for years. 

 

Pennsylvania's Finest - Brad Penn motor oil - Has all the Zddp YOu need and a beautiful Green Color to monitor the use.

The Green Oil.

kendall 20 - 50  has zddp

I am a brit transplant also, living in New York. Oil Change with a good quality 20W50. Castrol SYNTEC Classic 20W50 is discontinued but has been replaced with Castrol Edge 5W-50 which contains ZDDP. It says Classic Cars on the packaging, heres the link for more info:

//www.castrol.com/castrol/genericarticle.do?categoryId=82915470&contentId=7032644

Let the fun begin.  This topic comes up every so often and you'll hear many many opinions (and reasons for them).  I'm not an expert,  but have been happy with Vavoline VR1 Racing 20W-50W (ashless anti-wear additives and ZDDP at higher levels). 

//www.valvoline.com/faqs/motor-oil/racing-oil/

The CA issue is not 1975, it's actually post-1968 that's a problem. It needs to be pre-1968 to get past EPA checks at initial registration, then after that, if it's pre-1975, you don't need it smogged. I looked at bringing a 1972 into CA, and would have still needed to spend about $6k to get it EPA certified by the one place in CA that is allowed to do so (another one in northern CA is certified to do the check, but is not taking orders anytime soon).

That said, depending on where you are in CA, be sure to be good with your neighbors. If someone reports a CA resident as having an out-of-state licensed car for more than 2 (or 3?) months, there can be troubles too. I forget the details, but it's something like you're supposed to register it in CA within 30 days of having a permanent residence location, or starting a job.

Some areas won't be so bad, some will; in my neighborhood, I was preparing a car (not a mini), in my driveway, that didn't have CA plates on it; someone complained to the city, the city drove by my house, apparently multiple times over 3 weeks, taking pictures and "building a case". Then, I was notified that if the car wasn't removed from the premises within 48 hours, they would file a citation against me. They didn't know how much the citation would be, but the administrative fees would be $2500 before even determining the citation amount.

 

Plus...the auto "GTX" doesn't have the zddp....if you're concerned with that....the Grand Prix does

I've been using Castrol 20/50 4-stroke motorcycle oil. The gallon tends to be a buck or so cheaper than the non-motorcycle version.

You may pick up a horse or two, but you are compromising a high revving engine with a light weight synthetic (?) oil AND....5w-20w does not protect those meshing gears in the transmission when that oil is hot.  my $0.02.

I use Castrol 20/50 with 1 quart of Lucas oil additive, every 3,000, miles, for the last 65,000 miles.  Eventually, we all do what works for each of us.

If you can't find VR1  ( I can't ), then Castrol Grand Prix motorcycle oil 20/50 is readily available

Mobil 1 5W-20W per articles here and elsewhere.  Thinner weight oil for our engines, I get a noticeable bump in power.

I already ran into the CA registration problem. My Mini is a '87. Thank god I have a daughter in Mississippi.....just saying

+1 on VR1 20-50

Beware registration issues in CA if Mini is newer than '75