I am thinking about going 10-40 but what about 20-50?

Also, should I add ZDDP in a rebuilt engine or is that only needed for an orig. motor?

Thanks.
I have searched a bit on the forum but still have these Q's from what I have read.

 

10-40. 20-50 is pretty thick.

 

You use 20-50 when the motor is shot and you don't want it to smoke as bad when a buyer is looking at your car.
10-30 should be fine, but try 10-40 as stated if your motor is wearing out.
Lynn

 

The older engines had a lot looser tolerances than modern engines do.
20W50 may sound heavy by today's standards, but not by engine standards of the 60's & 70's.
In all my older performance Mustangs I ran 20W50 Castrol GTX.

I had a high perf Trans Am (450hp). When I drove to work (90 miles) the oil pressure on my mechanical gage would gradually drop down to around 20psi.
I tried several different oils and they all did this until I tried the Castrol GTX. The Castrol oil never thinned out, never lost viscosity due to heat and never lost a single PSI the whole trip.
Castrol GTX in my opinion is the best oil for a 60's mustangs up through the Fox body 302s.

Note: The newer OHC engines are another story, the heavier weight oils are usually not a good idea with those.

 

Thanks for posting this, very informative! I ran 20W50 in my 71 302 because it had an annoying valve tap, but I mean DAMN that thing ran like a dream with that stuff regardless.

 

I had a 71 Pont. GT37, 400, 4 sp. Always ran Castrol 20/50GTX. Too many Sat. night races to even remember. At 190,00 miles I sold it, and had never touched the motor. (unless you count the blown up clutch). For some reason I quit using it with later cars. This reminder may make me re-think that.

 

out of curiousity what pressure were you getting with the 10-30 cold/hot and also at 1000 rpm and 4000 rpm (or some low and high rpm point)?

 

i seriously doubt 10-30 is too thin... where all clearances checked during assembly?

 

I had a tired old 3002 in Louisiana and would run 15-40 usually, and straight 40 weight during the hot months.

 

I use 5W-20/30, should I be using 10W or even 20W? 302 with Performer RPM heads/Cam.

This thread is a great read btw.