Tom,
Let's see now, you used 20w-50, engages great, sometimes disengages great,
sometimes not? The thicker 20w-50 builds pressure quicker, hence the very
quick engagement. But, the 20w-50 also bleeds pressure off slower, hence the
sometime slow disengagement.
I've never had one that I had to let off on the throttle to disengage. That
sounds like an issue with the sliding member, maybe worn splines or
something. It should snap right back into direct drive almost simultaneously
with flicking the switch. Remember, the early cars even had an optional
vacuum switch to automatically shift down to direct whenver you stepped hard
on the accelerator, almost like passing gear in your automatic family sedan!
At any rate, there are more important issues to debate than oil viscostiy in
MGB overdrives. Your gearbox will survive with 20w-50. Admittedly, I've got
an MGB with 20w-50 that I put in it after I rebuilt it, and that's been
almost 10 years and 75,000 miles ago! The car that I mentioned in my earlier
post would not shift into overdrive. I suspected a pump pressure problem and
drained the 30w and put in 20w-50. Still, no overdrive. I removed the valve
body for inspection and found the problem there. Got the valve body parts in
correct order and orientation, and WHAM-O, overdrive. Then I replaced the
20w-50 with 30w, just to play it ultra-safe!
Have fun motoring this weekend!
Eric
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