Mineral oil is the best for all variants of fieros. And yes, as Sarvajit mentioned, 20W40 is the recommended grade although you may use 20w50 mineral for hot climates.
The one you have mentioned is going next in my bike as right now I'm using Shell Advance AX5 20w40 and it's superb! So, I can only imagine how good 20w50 will be.
You can also 20w40 mineral oils by Motul, Gulf and Elf with good results as most members here have used in the past (I did).
Semi-synth seems to seep out of the crankcase a bit and full synth becomes like water and disappears Doesn't make sense to use them for financial and mechanical reasons...

can you plz tell what oil grade Suzuki recommends for the Fiero?. are the oil grades same for the TVS Fiero, TVS Fiero F2 and Fiero FX?. mine is a 2000 Suzuki Fiero ( first version ).

so even a semi-synthetic oil is not good for the Fiero?. ( Shell Advance AX7 and the Motul 5100 )

if mineral oil is the way to go this would be my choice....Shell Advance AX5 20W50

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I guess you can use the original part of the crankcase which has the number. AFAIK you can split the crankcase.

My advice: Use 20w40 mineral oil Gulf Pride 4T Plus or Motul. Don't experiment with synthetic oils unless you want a blown oil seal or slipping clutch.

What will you do for the change in engine number then?

Will be sourcing the Apache bottom half with 5 speed gear box. So will fit the Apache starter too.

If you're putting the 5 speed g'box of the apache rtr, then you are gonna have to change your engine casing too. Because I don't think it will fit in existing fiero casing.
Did you source the fiero starter or an apache rtr one?

So I've received a lot of parts. Waiting to get home. In addition to what's already mentioned the bike is getting a 5 speed gearbox and electric starter.

If you give the bike for one problem you will get it back with two from the mech.

I also thought of an engine replacement by the RTR's (a used one of course) but the asking price was too high to settle with the same, after that never had time to hunt stuff.

I'm now reading this entire thread for the 2nd time trying to gather enough information. Trying to compile the following information so I can go armed with it to the mechanic.

1. Disk Brake: For now decided on a Pulsar unit. I gather from the thread that I need the following:
- Pulsar Classic Right Lower Fork tube
- Disk
- Calliper
- Hydraulic pipe
- Brake lever
- Hydraulic resevoir
- Fiero F2 Hub (Can I use a Pulsar Classic Hub?)
- CBZ Classic or Yamaha worm
- Anything else?

2. Big Bore: Tentatively decided on the RTR 180
- Bore kit (Bore, piston, rings and gaskets)
- Cam Chain tensioner
- Stock head to be matched on the lathe (can someone post a pick of a stock head and one matched to the 180 Bore)
- RTR head is another option (photo please so I can see the difference)

62mm bore= 175cc.(RTR160 bore kit)
62.5mm bore =177cc (RTR180 bore kit)
63.5mm bore = 183cc (CBZ classic piston)
65.5mm bore = 195cc (ZMA piston)
67mm bore = 204cc (P220 piston)

3. BS29 Carb: Already know that I have to buy it from Bajaj or the UCAL distributor
- Do I need to change the jets?
- Intake manifold of the RTR 180

4. Swing Arm:
- Victor GX (1240mm)
- Victor GLX (1260mm)
- Fiero F2/FX(1270mm)
- Apache 150(1260mm)
- Apache RTR 160/FI(1300mm)
- Flame (1320mm)
- Apache RTR 180(1326mm)
I think the Victor GLX one fits without any mods right.

5. Oil Cooler:
- As per the Apche thread
- Edit: Just read that Gixxer has done it on his Fiero

6. TCI Conversion:
- TCI unit= 650 Rs .
- Ignition coil = 450 Rs.

7. Exhaust mods

8. Cosmetic mods

9. Other minor mods like FZ handle, rearsets (what exactly are those),

10. Equivalent parts for the Fiero from other TVS or non TVS bikes to replace hard to fin Fiero parts or to improve the bike.

I would appreciate if everyone could contribute info and one person compile it all into a sort of a guide listing each and ever mod that can be done to the Fiero in the following format:
- Name of the mod
- Brief description
- List of parts with approx prices
- Procedure and labour costs

This can be a pinned post at the begining of the thread which can be a quick reference guide to anyone wanting to do it.

And once more can someone Please Please Please share a PDF/Scanned copy of the owners manual and service/workshop manual.