I have a problem: Did I put the wrong oil in the tranny or what. I wanted to make sure everything was up to par for the 15 T I have recently purchased so I changed the oil. before that the tranny seemed to be working fine but after I installed very expensive Castrol motor cycle 10w-40 wet clutch full synthetic oil my tranny is now slipping so bad I couldn't even pull up my driveway which is not much of a angle.

I would change to a straight 10w-30 oil and leave out the super douper slick synthetic stuff.

I and several of my friends have been using Mobil 1 full synthetic motor oil for years with no slipping problems. Two of us are using 400 clutch disks.
Suggest changing the oil to conventional 10w-30 and drive your 15T around on the flat and see if it improves.
The three of us using Mobil 1 are using original clutch springs. Could your clutch spring be weak or does the clutch plate need adjusting to put more pressure on the disk pack?

I would drain it out and get some 10w-30 from auto zone and use it. If your T's transmission was working before you changed it the problem could be your high grade oil is to slick?
The opinions are all over the map about which oil to use in a T.

I personally don't think its necessary to use expensive oils in T engines.

Any standard grade 10-30w or equivalent is much better oil than any oil that was used in the Model T era and will work just fine.

Let the other opinions begin.

The cheapest 10W-30 or 10W-40 you can find that has this on it.

I use a non synthetic motorcycle oil (Motul) and it works well. Since it is formulated for machines that the engine and transmission use the same oil and wet clutches it makes sense to me. The full synthetic could be just a little too slick for the bands/clutch. Besides, at the rate T's leak, why waste the synthetic.

Marvel Mystery Oil. Or wait a minute. No, for Pete's sake don't do that.

Uhm, ah, 10W-30W. That's what you want. The cheaper the better. The cheap stuff won't hurt the lakes and streams as bad as the expensive stuff when your Model T leaks it out onto the ground and it flows into the storm sewers.

Use the Marvel Mystery Oil on your etimer and waterpump.

I have found that specialized, oil only places sell quality brands at super low prices. Way less than even the worst oil at regular stores. Buy from there and change it often.

Or you might try adding some bituminous asphalt roofing tar to that slick oil to "thicken it up."

Mike Walker

Mike: How much is "some". I want to make certain my mixture is correct for the application intended.