Tired engine? 2001 Vitz RS

[Mehavesnoname]

Hey.

I've got a well used 2001 Vitz RS that i bought for cheap a few years back (exterior in rough looking condition, paint/clearcoat coming off, front bumper damage, wheel coating eaten away by something etc etc) and it had high km's when i bought it (305,000km/s, now up to 335,000km/s).

The first things we did were replace all the spark plugs (to decent ones, not the cheapest available), replaced air filter, changed oil and oil filter plus a couple other things. Since then we've also replaced the fuel filter, and have done multiple oil changes in the past 30k km's too.

We've changed it to thicker oil recently to try stop it from going through so much (needed to keep topping it up, more than both my other cars combined). Started using 10w/30 synthetic (?) and now using 10w/40 semi-synthetic. It seems to have mostly curbed the excessive oil use.

But the main reason for this post is to hopefully get some sort of idea what is causing the tired feeling engine.

Basically the symptoms are;
- A grinding/mechanical type sound when putting decent load on when accelerating (half throttle or more). I've driven many cars (albeit with lower k's), and it's not a common sound. It's quite quiet, you can't hear it with windows down or radio on, but it's there.

- Inconsistent power delivery. Sometimes it's sluggish power delivery (with little surges thru the rev range, like it's turbocharged etc), and sometimes it's really zippy and rev happy. It still seems to be random and not really caused by anything in my control?
Like i can floor it in second gear one minute and it's nice and smooth and linier, the next time it'll be "nothing-nothing-nothing-POWEEEEER-less power-POWEEEER" etc. Basically it feels a bit like my turbo Nissan, but with less of a kick when the turbo kicks in.

- The aforementioned oil burning issue, for which our mechanic suggested thicker oil.

Firstly, i know i shouldn't expect much from an engine with such high km's, and no idea how it's been looked after by previous owners, but basically i'm hoping to find out the possible reasons for its inconsistent power delivery and roughish sounding noises when under heavy throttle.

I very nearly bought a 150k engine the other month, but opted not to only because the car is still reliable (*touch wood*) and wasn't sure if fixing this power problem would be an easy fix, or an engine rebuild. If the latter, it's significantly cheaper to just chuck another engine in it ($8,000NZD+ rebuild compared to $2,000 NZ incl. engine).

At the end of the day it's just my daily driver and our road trip/gravel road car, so it needs to be reliable. Which is why i wasn't bothered about the exterior condition, as it's good for parking in supermarket car parks where no-one cares about opening doors into other cars.

It is a tough wee car though, and it loves gravel roads and other such adventures. It's a very fun car to drive despite the low power, and the interior is in quite good condition surprisingly (aside from wear and tear on gear stick and steering wheel), which is why i intend to ignore my mechanic's advice to scrap metal the car when the engine dies.

Anyway. Would be good to know if i should fix it or just do an engine swap or not if another one in the 100,000km range pops up for sale again.

Cheers.