I have a 67 F100 with a 390GT FE engine that I rebuilt and now has around 12,000 miles on the motor. The problem is I think I screwed up the break in period by using synthetic oil after 500 miles, not giving the motor long enough to seat the rings properly.

Problems with oil consumption (goes through about a litre every 2000 miles), oil turning black, compression varies and doesn't seem where it should be
(anywhere between 160-180 lbs, where used to be 175-180lbs before rebuild), leaks from gaskets.

I asked a local engine builder if it was too late to seat rings. He suggested running a high detergent dino oil to rinse the synthetic from engine parts and allow the motor to break in. I was wondering if anyone here has any suggestions on what brand/type/grade oil I can use to rinse away the affects of the synthetic. Would a diesel engine oil work? It currently has Valvoline full synthetic 10/30 in it.

 

The compression is probably lower because a different head gasket was used on the rebuild. It probably comes up every so often due to carbon buildup in the chambers.

Unfortunately, if the rings haven't seated by now they probably never will. The finish hone leaves the cylinder walls a bit rough to aid in breaking in rings. Once that's gone, there's no turning back.

You can try a different oil. I'd look for a diesel rated oil, with the CI-4 rating in addition to the SL rating. I use Mystik JT-8 "Super Heavy Duty" 10w30 in my 390. Some others are Shell Rotella T, Chevron Delo, Valvoline Premium Blue, Mobil Delvac, and others.

Really though, one litre in 2000 miles isn't that bad, especially for an FE. There are some new engines that dont do as good. I'd try the other oil and see what happens.

Edit: What leaks do you have from gaskets? Maybe that's where your oil is going?

 

It doesn't leak that much, accept for rear main seal may leave a spot on the garage floor, its more like weeping or wicking around oil pan valve covers, intake, etc. I show the truck and have to constantly keep it spotless by cleaning off any residue.

 

A local engine rebuilder swears that he gets much less oil consumption with straight 30w dino.

I know that Briggs & Stratton also recommend straight 30w in lawn mower engines because of less oil consumption than when 10w-30 is used.

 

Well my way of thinking is, if it leaves one spot on the garage floor, it's probably leaving 100 or more on the highway. That does add up.