Have a 1987 Toyota Hiace Diesel LH series Campervan. Last couple of years been topping up about 1.5 litres oil every 1000kms. Have been using 20-60 Penrite HDR Diesel oil. Unknown to me my mechanic has been using 15-40 oil on every 5000kms service. Is this the problem - mixing grades? Engine runs and sounds great - no blue smoke on start up etc. No blow back. Wet and dry compression test showed no problems. I am an elderly lady and no one can come up with any answers. Please can anyone help
that is 1.6 quarts per 621 miles or 2.6 quarts per 1000 miles. That is on the high side.
I doubt that the problem is mixing grades.
The oil consumption is high. It’s also quite possible to have great compression readings and still have an engine that consumes oil past the piston rings.
A problem in this area is usually caused by oil control rings that are stuck in the pistons. That is usually due to age, overheating, extended oil change intervals, etc and the only cheap easy cure is an additive for the motor oil along with a prayer.
Valve seals can also cause oil consumption problems. These are more easily replaced but there is no test for them. That’s a replace and pray thing also. 
Did they happen to provide both the dry and wet compression test numbers to you?
How many miles/Kilometers are on this vehicle.
I don’t think oil grade would cause excessive use. There my be a leak and some are only when the engine is running, so you wouldn’t have a stain on the floor.
For a 87 it may just be weak piston rings. That, would call for a engine rebuild, or newer engine.
But as my dad said once " you can buy a lot of cases of oil for the price it would cost to rebuild the engine." I’d just keep an eye on it when you’re on trips and check it every fill up or 400 miles…whichever comes first.
Yosemite
Seeing as how this is apparently a diesel I’m sure curious about the compression numbers.
I’m not familiar with this model at all but maybe oil loss could be through a mechanically driven vacuum pump or turbocharger if so equipped with it.
I’m not familiar with this engine, is it turbocharged?
Do you have the owner’s manual or service manual for this vehicle? That will provide the ultimate authority as to what the proper oil is, but 1987 was a long time ago and I can tell you that 15W40 Delo, Delvac, Rotella, or a few others were the go-to diesel oils of that time period. I doubt the problem is the type of oil.
On an old diesel like this, I’d really want to know where all that oil is going. Diesel runaways are no fun!