I am almost done with my 85 Sovereign engine rebuild (documented in another thread) and will be test running the engine on an Easy Run Engine Test Stand (more on that to come on the other thread).

For 100,000 miles I ran 20W-50 Castrol conventional on that same engine. For break in purposes, is there a particular brand/weight of oil that I should be using? Its time to fill the crankcase.

Thanx.

 

You are right in the range with that Castrol. Maybe Shell Rotella 15W40.
Something with sufficient ZDDP whatever you use.

You want to change it out in the first 500 miles as cheap insurance
against build debris.

 

Just use the conventional oil recommended by Jaguar. Today's conventional oils are great and backward compatible for use in as built standard performance older engines. Since your engine most likely uses a standard valve train any brand standard conventional oil will be fine. If you had a high lift cam, then maybe a high ZDP break-in oil would be called for.

The only suggestion I can add...Break in the motor and change the oil and filter with in the first 20 minutes of run time. There is a lot of initial wear soon after the fist run.

 

The oil you've been using is just fine. Carry on.