High mileage oil choice?

My 88 GMT400 has at least 400k miles on it. TBI 350, flat tappet cam. It's driven at least weekly, but isn't a daily. 

It usually burns one half to one quart between changes, which occur about every 9 months. Odometer doesn't work, probably 1000-2500 miles between changes. It runs amazingly well, and I'm considering using it to open trailer my autocross miata.

It puffs a bit of white smoke from leaky valve guides at startup, which probably accounts for nearly all of the oil loss. I've decided to not change out the guides. I'm afraid to mess with it, due to the age and milage, and the fact that it still runs perfectly.

I've been using high mileage mineral oil Pennzoil 10-40 with a bottle of Wynn's. I have changed an older vehicle to synthetic with subsequent oil leaks. I wish to avoid that experience again. Based on the current article on oil, I'd like to stop using the additive, and avoid using racing or diesel oil. I also don't want to change to synthetic.

How's the oil pressure at hot idle with the current oil?  

Good question. It's hard to tell with the stock gauge but I think it's pretty reliably about 15.

It's never dropped low enough to turn on the engine light. It used to run a bit lower, but after I added a 40K transmission cooler it stays pretty cool all of the time.

Years ago I blew the engine in my E30 track car at Tremblant and slapped a used M20 into it for the rest of the season. Turns out the engine was in really bad shape with low compression and tons of oil consumption.

I ran one quart of Lucas HD Oil Stabilizer (basically molasses) and filled the rest with whatever 20W50 was cheapest at Canadian Tire that week (maybe Kendall GT-1 or Castrol GTX ... I really can’t remember).

Oil consumption was dramatically reduced. The engine survived. Despite the “Bob is the oil guy” naysayers screaming about foaming or whatever ... nothing bad happened. I ended up using that engine for another year before finally replacing it.

Anyway, that’s one datapoint in the plus column for Lucas HD Oil Stabilizer.

The worst thing oil can do to your engine is not hold its bits apart from each other. With that in mind, I just run thicker viscosity oil in my worn out engines. When they're making <10psi hot idle on 20-50 I'm pretty much resigned that i have to get in there and replace bearings.   Sounds like your engine is doing well for its age! 

Maybe I'll step up to the 20-50 for the next time, leave out the additive. 

It's beautiful out today, but we're about over our cool weather here in Florida.

Any opinions on brand, etc?

In a Florida climate and with 15 psi at hot idle on 10W-40, I'd just run cheap dino 15W-40 HDEO in it.  I know you said you don't want diesel oil, but it's probably the best pick for a non synth oil in a situation like that.  

I had 93 with high mileage like yours. I ran Lucas stabilizer and Castrol  20w-50 in it for years. Anything less then that it would leak and not  “feel” right. I also ran with gutted thermostat during hot summer months..

rslifkin said:

In a Florida climate and with 15 psi at hot idle on 10W-40, I'd just run cheap dino 15W-40 HDEO in it.  I know you said you don't want diesel oil, but it's probably the best pick for a non synth oil in a situation like that.  

That's what I do with mine, either Delo, Rotella or the Mobil HDEO. The formulation helps the cam live a bit longer too i think (flat tappet).  Best thing is you can get it almost everywhere, even in convenience stores if you run low.  And it's relatively cheap..  May be a good idea to check the valve stem seals, replaced the ones in my 91 and my oil consumption dropped to less than a quart every 3000 miles. The old ones are probably done if you're seeing smoke on start up.   

I'm seeing five gallon buckets of Shell Rotella T4 15w40 for around $60.  I think something like that would be the ticket.  And I think I'd run the change interval out longer, to something between 3k and 4k miles or once annually.